It's completely random - some people get it fast and some people take forever. The wiki says the chance is one in 20 but with how random chance works just because the chance is 1 in 20 doesn't mean the actual wait time is 20 nights. Some people get it first try, others have to wait one hundred days or more. It is actually a really fascinating example of how probabilities cant predict individual cases but that's a whole bunch of math nerd stuff I won't get into.
Basically, there is nothing you can do but sleep until your spouse asks the question. There is no way to control the outcome, you just have to keep trying.