Or does that only happen when you swap to 12/12 for flowering? Because I'm trying to grow a motherplant that I clone forever but I'm not sure if they will sprout their telltale signs in this timeframe?
No, typical pot plants won't show sex under 18/6 unless there is a problem like low light levels or high Potassium levels. They may preflower under somewhat less than 12hrs of darkness (e.g. 11hrs), but in my experience need more than 9hrs of darkness to do so. The absolute minimum amount of darkness required for preflowering varies with strain. I've vegged clones under 15/9 without any flowers. A full 12hrs should be used for actual flowering to avoid issues like decreased yield, slow bud growth & slow ripening. This doesn't include autos. I've vegged mother plants for years without any flowers.