You can never have too much contingency built in. Depending on the season, I'd allow a minimum of 20%-%30 cost over run with a 50% over run on time.
Weather is something you can't control, trying to do a CPL nav in the middle of winter can be fun or flying IFR with the freezing level at ground height can be frought with traps. CPL nav in summer with a few fires burning tends to knock the vis on the head too.
A/C serviceability.
Instructor illness/availability
Your health/availability.
Exams...
Drop one or two of them and your time plan goes out the window unless you've got some time spare as such.
Good luck.