How long to cook boneless chicken thighs at 350?

This depends upon the amount of chicken you are cooking, whether it is whole or cut into pieces, the thickness of the pieces, and how reliable your oven is in maintaining the temperature you have set it at.

You will find that by setting the temperature to such a high setting, the outside of the chicken may be very browned long before the center reaches a safe temperature for eating, and the food may become dry. Look online or at a recipe book for suggested temperature and times, and then begin to test the chicken with a meat thermometer when it starts to look nicely browned. This will tell you if it has reached the "poultry" temperature. If it has not, you may need to cover the chicken with foil and lower the temperature so the outside stops browning and the inside has a chance to cook thoroughly without drying out.

Achieving moist and tender chicken usually is done by cooking it more slowly at a lower temperature. You can start with it covered with foil or a lid, and remove the cover when it nearly done so it can brown up nicely