How tall are your speaker stands

What is the average height of stands for monitor speakers?
Would 30" stands be too tall?
The speakers are 12" tall and not too heavy.

It depends on speakers. Some speakers should be placed with the midrange at the ear level, some should be lower, some should be higher. You need to find out from the speaker manufacturer what the recommended hight is.
I've heard tweeter at ear level. Mine are 24" (Osiris). I suppose if you have home theater seats instead of a couch, they could be higher.
Its tweeter at ear level but many variables can enter in. You could have the speakers tilted up or down if at wrong angle.
Although tweeter at ear level is the general rule, experimentation can work wonders. I had a pair of Silverline Audio SR-11s sitting on 24 inch stands with tweeters at ear level. The sound was good but if I slouched down the sound was better. I moved them to taller stands which raised the tweeter about six inches. Huge improvement.
More recently I had the opposite experience with PMC DB1+ speakers. They started out with the tweeter at ear level but after some experimentation I found the sound improved with the tweeter about 4-6 inches below ear level.
Experiment to get what suits you best has become my rule.
I have found that for me 42"-44" ( tweeter height ) works best......I have 20" Stands and the tweeter is 22" ( from bottom of monitor cabinet ..It all equals aprox 42" height of tweeter ( center ) from floor .......42" is aprox ear height from my listening chair......
Tall enough to get on all the rides.

Stand height depends on tweeter height in relation to the listening position. The tweeter should be at approximately ear level.
Timrhu is right, it depends on the speakers.

Generally speaking, I like tweeters to be above my ear level. This usually produces a more spacious sound minus the brightness. Another advantage is higher off the floor and clean up the bass even more.

But there are exceptions. My large speakers sound better with my ears at tweeter level.
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I agree 100% with member Entrope. If anything you can always
change out the listening chair/couche(s) if needed. In my case the stands cost a lot more than the chair did! LOL
BTW- get an adjustable type stand 20-24" should do the job nicely.
Tall enough to reach the ground.
There are many full range speakers with tweeters positioned higher than the ears level. Also, in many live performance in small rooms, the singer's voice would come above the ears level when you sit in a chair.
So, I am not sure how accurate the general rule (tweeter height == ears level) is. But, you may not have it so higher or lower than the listening level. ;-)
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Thanks for the input. It looks like I need at least 30" which I question the stability as I never see commercial stands this tall. So it looks like either 24" stands on stands or DIY.
Perhaps 24" stands on paving stones or slabs from your local garden shop. They are usually about 2" thick, inert stack without sliding and come in sizes up to 24" x 24".
As an alternitive place the woofer 38percent of celing height and tilt the speaker so the tweeter is aiming at your ears. Maybe turn the speaker upside down. IDK just feeling kinda radical today.
Speaker stands should put the speaker's tweeter at ear level in a seated listening position.