Repeat after me, kids! In Gen 5, IT IS NOT WORTH TRAINING A GROWLITHE TO LEVEL 45. GROWLITHE IS A BAD POKEMON!
Growlithe is frail and has middling attacking stats. It is useful until you defeat Colress in Castelia City, after which it can be overpowered by most Trainers. Despite what people on this site like to say, it is NOT worth waiting for Crunch, Outrage, and Flare Blitz when you could be using a good Pokemon instead.
The best level to evolve Growlithe is Level 34. This is still far past the point where Growlithe is useful, but this is tolerable because it allows early access to Flamethrower and ExtremeSpeed. Neither Growlithe nor Arcanine are particularly useful against Clay, so you're only really missing out on a strong team member against Elesa. Comparatively, delaying to Level 45 makes Growlithe useless against Skyla, whereas Arcanine can put in reasonable work (particularly if you teach it Thunder Fang).