I saw a comment on one of the previous rationality diary threads about starting a meditation habit by meditating for two seconds per day and later had the realization that when starting this particular habit it might be better to track different units at the beginning than you ultimately want to end up using (and can't find the original comment now). So when starting, you might just practice one breath mindfully, then increment the number of breaths. When you hit some set larger number like 8 or 10, you might switch over to counting blocks of breaths until that becomes disruptive (probably more than 2 blocks for me). Once you hit this point you're probably up to about two minutes, so you could switch over to using a timer and tracking minutes.