> get it to something you or a capable colleague can hammer out in m minutes. Give the candidate 3m minutes to solve it

I think they're using "m" there as a variable, to frame that you should give the candidate 3 times however long it takes for an insider to solve it.

> Typically this means it should take you 15 minutes to solve and give them 45 minutes

They mean 3 x m where m is how much time you think it would take for you to solve; so the example is: if it takes you 15mins, then give the interviewee 3 x 15 = 45mins

3m minutes is already shy of 6 years. I know that tech interviews are long but this is excessive.