Each vitamin or mineral has its own half life. For some B's, in the right circumstances, we are talking days or weeks, although typically DS people do not have a problem with B's. For some of minerals, we are talking years. Just from watching the super non-compliant people come through on this board, it seems like many of them last a year, maybe two before they notice the malnutrition catch up with them and their world of troubles begin. Oddly, The American Bariatric Surgery Association have all sorts of statistics showing DS problems (zinc and A especially) can start around 4 years, and that is with people already supplementing the crap out of themselves. I don't know if that is a magic length of time, or only what they studied.
Also, your general state of health is important. As all people age, they lose the ability to absorb nutrients as well, or specific diseases will cause changes. So you may be fine for 10 years and then some other developing health issue will throw off everything and lucky us will catch it a year or two early because it will first show up after careful study of our bloodwork.
This is a huge advantage we have over "normals".
Or any number of seemingly unconnected events could trip you up, like you could move. Where I live there isn't selenium in the soil, so I watch that number, but some of you live in super plentiful areas and you would always be fine without any supplementation of that mineral.
I'm almost twitchy about this subject right now because I'm a month late on my 2 year bloodwork. But I keep screwing up, like forgetting my K's for 3 weeks and then after 2 weeks of doing it right, forgetting all my vitamins on a 4 day trip to Canada. I want to have a few weeks of "normal" compliance behind me before I get my tests so that my results will tell me about the success of my overall program, not that I'm an idiot sometimes. I already know that!!!
Good Luck!