How long does the immunity last after post exposure vaccination against rabies/?

2 years: Immunity induced by rabies vaccination lasts at least 2 years.

>10 years, but...: There is good evidence to suggest that the rabies vaccination provides long term immune memory (>10 years). Whether that response would be sufficient to completely protect someone from a new exposure 10 years down the line is somewhat vague. Because of this, it is recommended that you get a booster if you are exposed to rabies again (2 shots instead of initial 4).