Interesting, mornings mostly, often after the gym or working out (such as running), many times after being out in the sun, lying out at the beach or by the pool (often times in the nude since I have a pool at the home), sometimes through out the day, during business travel prior to bed, and to relieve stress (I think its much better than medication).
On a separate tangent I like to masturbate with my lover watching her watch me or mutually enjoying each others solo play is an incredible turn-on or aphrodisiac that usually leads to some GREAT sex. I like to get on her in the Video Chatroom too. It is a turn-on to play for an audience and it doesnt matter the viewers sexual preference or if they have a cam either. If they like the show and make comments or if they have a cam and you can see their excitement too its a turn-on for sure.
Having grown up in Catholic Schools and being raised a Catholic the unfortunate stigma they put on it may have repressed the enjoyment early on but once the reality of how good it felt was out there was no going back. I think every healthy person should enjoy some private time exploring their own sensitivity and sexual self as it only makes a better lover.