How long should it take a platter to get up to speed?

I have an upgraded Acoustic Signiture Final Tool with 2 motors. It takes more time to speed up now. I've had it 3 years. It takes about 15-20 seconds. Is this a normal speed or is my belts slipping ? I washed them with Ivory dishwashing soap and put some pure talcum powder on them but it did not make a difference . I have tried different positions farther and closer. Maybe I should get new belts? Or is everything normal. Thanks. Mike