I'd suggest that on an otherwise unrestricted route,that you plan for a cruise segment of at least 10 minutes, you don't really want a parabolic up-down lob (you were talking B737 sized aircraft)..
Of course, the route may be too short for a 10 minute cruise segment, but that is prettty rare, .
The 10 minutes in the cruise in practice allows for all the necassary admin stuff (crew discussions, company chat, 'paperwork', and importanty a cup of tea and time to catch your breath).
That would mean on anything over about 50 minutes you have time to get to FL350, cruise and get back down (subject to any other constraints).
I've been retired 11 years, but I'm sure the basics haven't changed that much in the intervening years!