Plan formula review sessions.
Memorising formulas reliably needs several spaced encounters rather than one long session — five repetitions is a common working figure for durable recall. Multiplying formulas by repetitions gives the total encounters, and converting through seconds per repetition gives a daily minute budget. New formulas must be introduced early enough for their repetitions to fit, which is why the introduction rate is spread over the days minus the repetition count.
Practice volume
Total repetitions = formulas x repetitions each; total minutes = repetitions x seconds each / 60
Daily schedule
Daily minutes = total minutes / days available; new formulas per day = formulas / (days - repetitions)
Retrieval strengthens memory most when it happens just as recall is starting to fade. Massed repetition feels more productive but decays far faster, which is the well-established spacing effect.
Even then, knowing them fluently saves lookup time and, more importantly, lets you recognise which formula a problem calls for — which a sheet does not help with.