It is less if the girl has higher stats. It is maximum 14 days.
The right time seems to be asap. There are a lot of diamonds per girl. 10-15 diamonds should be rather easy. And if you you manage 15 wins against different girls, there are 25 diamonds per girl. If you can buy a warp x8 with those diamonds, the lost reset time is well spent. Furthermore, resetting with more warp is better.