A big phat no-no to using bots to make any design decisions, in my opinion. They can't substitute human players, not by a long shot.
Quote from SquadBots are useful for checking (early development) chokepoints timings and for checking angles and generally how "fun" a specific spot is or can be.
As soon as you think your layout is getting pretty decent, you should organise a playtest with human players though.
Timing depends. Bots can be incredibly stupid sometimes and if your paths involve anything other than simple, flat floors (jumps, crouchjumps etc) it can be a hassle.
I recommend using a stopwatch and running to a location as both teams. For very exact timings, host a local server and have a buddy join, then both head to the same path and see who arrives first and by how much etc.