Generally weekdays from 6am - 8pm are good times to send a survey.

Depending on your audience you may also see improvements by sending earlier in the week and earlier in the day. Mondays and Tuesday mornings (7am - 10am) being the best.

If your survey is related to an action like a purchase where you want to get their feedback while it's fresh in their mind, the sooner after the action is taken the better.

If your survey is related to someone's overall experience with your product, give enough time for them to have formed an opinion, at least a few weeks.