depends on how fast they are developing. the Trichocereus are doubtless going to grow alot faster than the pelecyphoras, arios and lophs you have for example, and can be weaned off the high humidity a lot faster. in fact its probably beneficial to prevent them becoming elongated and leggy in the monsoonal conditions. basically it is important to maintain high humidity and to prevent drying out in the initial stages of development, after which you gradually reduce the humidity and ultimately acclimatise the seedlings to normal air conditions.
I have ario retusus and fissuratus seedlings that have been covered for 3 months+ now. they are starting to develop tubercles but are still tiny.
i kept seedling Epithelantha micromeris covered for more than 5 months, gradually poking a few holes in the plastic, then a few more in the weeks to come and so on and so on and now they grow under normal air conditions, but still in the propagator.
I have heard Blossfeldias can last covered for 2 years as they as so tiny.
i never worried about the algae, as soon as you reduce the humidity it disappears, although a problem with this i have heard is when you grow Aztekiums and blossfeldias which actually grow slower than algae and and is resultingly smothers the seedlings.