Here's what I've been reading here in the US:
How soon after starting PrEP will I be protected? How long do I have to keep taking it to maintain that protection?
SEXUAL EXPOSURE
Before: The best available evidence suggests that, after starting daily Truvada, it takes 7 days to reach maximum protective effect against potential rectal exposures, 20 days for vaginal exposures. Investigation into this question is ongoing, and some researchers tell us that 7 days is conservative and there may be very good protection after only 4 days.
After: It is advised that you continue to take Truvada daily for 28 days after your last possible exposure to HIV. If you have an active hepatitis B infection, talk to your doctor before you stop Truvada. Stopping Truvada can cause active hepatitis B to get worse quickly.
Note for Transgender Men: we do not have good evidence about how well these guidelines may apply to you. The guidelines came from studies that were limited to cisgender men who have sex with men and cisgender and transgender women.
Non-daily Dosing: In a study called IPERGAY, researchers found that non-daily dosing was also effective for some people. CDC has not endorsed the IPERGAY regimen and recommends daily dosing for PrEP. For more information about the IPERGAY study, please click here:http://on.fb.me/1bF9HhV.
INJECTION EXPOSURE
There is no good data on how long you need to take daily Truvada in order to have protection against an injection exposure to HIV. It takes 20 days of daily dosing for Truvada to reach maximum concentration in blood. However, even at maximum concentration, we do not know how well, or even if, Truvada protects against acquiring HIV from an injection exposure. There have been no clinical studies that examined this. Nonetheless, CDC recommends PrEP for people who inject drugs (http://1.usa.gov/1HfclIQ).
Sources:
http://www.cdc.gov/hiv/basics/prep.html
http://www.ncbi.nlm.nih.gov/pubmed/25409469
http://cid.oxfordjournals.org/content/60/5/804.full.pdf
http://betablog.org/iprex-update-with-robert-grant/