That's actually a good hack. I tend to take a similar approach, but using approximate days in a fibonacci sequence.

Start at the high end, will it take... years: NO quarters: NO months: erm... NO. weeks: maybe days: Unlikely hours: Ha. No.

So you end up with a range (days?-weeks-months). That's too broad, what could go wrong to avoid making it months long project (well we could investigate X, Y, and watch for Z). What needs to go perfect for it to be days? (well we could... wait, days is unlikely).

Those discussions about the high and the low to get "reasonable" confidence are super important.