This is a tricky question to answer. The vote build runs continuously 24/7. It takes a certain number of hours to complete each run and then variable time for each title to access the latest results. It also somewhat depends on which interface you use to access IMDb.
If you happen to vote immediately before a new vote build runs then your vote might be included within a few hours. If you happen to vote immediately after a build starts then effectively you have nearly twice as long to wait.
In practice this means anywhere between 4 and 12 hours.