All right...

# of words in 15 minutes
The rest is shorthand, but can be derived this way:
# of words/15 = words per minute
1667 is the number of words needed every day (minimum for 50k in 30 days)

Do you remember how to divide fractions? It is 1667/(#of words/15min) = 1667 * 15 / # of words

Brandiwyn🎶Sprinting kindly rounded 1667*15=25005 to 25000

Therefore, 25000 / (# of words) = Time to complete one day's worth of nano.

*There are always variances in this factor, but it is a start to figuring out how much time you need.