15 foot RCA cables out of TT with no pre amp?

Im very limited with the space I have. This is kinda limiting me to keep the turntable on a 15’ cable. I’m using a Pro-ject Carbon Debut Evo with an Ortofon  2m blue to an Onkyo stereo with a phono input. Cables are midrange, nothing special but not total crap. Just wanted to get some opinions or suggestions on this. How bad the loss is gonna be if there is any. 
You want your TT and “stereo” as close together as possible, and see how it sounds. Or, get yourself a secondary phono stage and put that as close to your TT as possible and run the RCA’s out form it to the Onkyo.
As a temporary set up, if possible.
Place the Onkyo as close to the TT as possible and use a shorter IC.
Does Temporary arrangement require a Longer Speaker Cable ?

If it works you can then Try the Long and Short IC configuration.

Using a separate Phonostage, as already suggested, with the Onkyo will also offer other configuration options.  
You will be screwing up the high frequency response of the cartridge using a long cable. There are many inexpensive phono stages you can buy. Put it by the table and run the long cable from it.
Keep your turntable cable as short as posable and get a phono pre with XLR output you need a phono for that distance 
Doubtful the Onkyo has balanced inputs. If it does, XLR outs are a good idea, but would need a new cable too, and they are usually more expensive than their similar quality RCA counterparts. At the level of TT etc. you could pick up a Pro-Ject phono box for less than 100.00 and be good to go.
So, if I’m hearing this correctly. I should use the 1.23m cable provided to a phono stage. Then after the phono stage, the long cable will suffice? 
"...So, if I’m hearing this correctly. I should use the 1.23m cable provided to a phono stage. Then after the phono stage, the long cable will suffice?..."

The phono output is at a low level, you want to keep that run short.   
The main issue is not only loss but noise you want to keep the connection to the turntable as short as you can due to these reasons try to arrange a way for that to occur so you do not develop noise or hum.
Any "loss' would be overcome a noise sddition. All you can do is try and see if you can live with it.
Yes, keep the TT output cable as short as possible. It’s carrying a very low level signal, and thus is more prone to noise and other issues.

On the other hand, the phono preamp out is "line level" and fine with a longer cable. I, too, prefer balanced connections if the gear on both ends supports them. If you can use them, you can have cables made up from Mogami W2549 cable and Neutrik connectors by a Pro Audio shop (if you are in the USA, ProAudioLA does great work) for about US$100 a pair or less, depending on length, of course. I found them far superior to $2000 "audiophile" cables.
That seems a bit too long.  Typically 5 feet is considered max, especially if you are using an MC but you are ok with Ortofon Blue since it is an MM cartridge.

The MOST important factor in a phono cable is the shielding and the extra capacitance.  When it is too long, the extra capacitance may cause it to increase the high frequency response (treble) to a point where it may be noticeable.  

Voltage loss is not going to be too much of an issue since there is virtually no current flowing in the wire, since the input impedance of an MM phono amp is about 47k and a few extra ohms from the cable will not make that much of a difference.

However, try it and if it sounds ok, you are done, if not, try and keep it shorter.

I would listen carefully to a very familiar lp with distinct lows, mids, highs. Then make a temporary short cable hookup, listen, despite theories, I doubt you will hear a difference.

let us know if you try this.
Most brands of TT have less than 1 meter of cable coming from the cart. I would recommend Shiit Mani to start with. At $150 it is a really great product. From the Shiit Mani you can run the longer cable to an input on the Receiver. Do not use the “Phono” input if you are using a phono preamp like the Shiit Mani. You can use an empty RCA input like AUX, Tape, or VCR since these are usually open. 
The effect of cable length is as a multiplier of the capacitance per foot of the cable. A typical MM cart has an ideal capacitive load for flattest response. You have to know the capacitance of the internal wiring of your EVO, subtract that from the target, and divide the result by15 to know what type if cable to use. Getting this info ain’t always easy. The recommendation of BJC LC-1 with an added ground wire seems pretty sound to me. 
I failed to mention the Onkyo receiver preamp has its own capacitance. If the EVO has 80 pF and the preamp another 80 pF you need a IC with next to zero capacitance...as the target is 200 pF for Ortofon 2M. Hence the recommendations for a closer placement using an outboard phono stage. 
If you didn't encounter noise/hum problem with 15 ft cable, increase the high by using the tone control on the Onkyo stereo to compensate the high frequency loss.
I am surprised no one else mentioned this, but not more than a few weeks ago, another guy asked about using a 22-foot phono cable.  You might search for that thread; all of the responses also apply to yours. Or was that you?
Cables are midrange, nothing special but not total crap. Just wanted to get some opinions or suggestions on this. How bad the loss is gonna be if there is any.
@lewm seems like OP already installed it and ask for opinions.
@lewm The TT is being shipped over now. Along with a Pro-ject phono box II. I wanted something to ground the connection, and by suggestion of the chat here; give the signal a little boost to the receiver. I’ve had the cables sitting there for a while, but now that I can see the pre-amp will take off at least 3 feet, i’ll most likely get a shorter cable!
Also, the post regarding the 22 foot cable was not me. Seeing how even 3 feet can make such a difference, I wanted to double check if subtracting 7 feet from that would make a barely noticeable difference or not.