18 months seems reasonable.
Getting all your ratings in 18 weeks seems crazy. What schools are offering that? Why don't the full time schools offer that?
I guess it took me about 9-10 months to get to 250 hrs (CFI/MEL). 40+ weeks. I have a hard time understanding how they do it in 16-18 weeks.
For example an instrument rating requires 40 hrs of instrument training. To achieve that in 10 days requires 4 hrs of instrument training a day. That's probably exhausting.
I max'd out at 29 flights in a month. Followed by 20 and 17 flight months. That fast track program is talking about doing 1.5 to 2 months of flying in 10 days. Ouch.