How long will a bottle of mederma last?

It depends.: It depends on application area. More importantly, note that mederma and the majority of topical treatment for scars have been shown to have no improvement on normal wound healing, (but it won't hurt.) vitamin e and massage have some very week evidence of benefit. Protection from the sun is the thing that has shown the most benefit- use and reapply good sunblock during the first year of healing.