That depends on the type of application.
If you lodge your application in person, you will be given a yellow receipt that states the date on which your decision will be made and when you can collect your passport.
If you have sent in your application by post then you should receive a decision within the agreed processing timeframe for postal applications.
Processing timeframes are indicative and can change during busy periods such as the summer months and in the run up to the Christmas period. Every Monday, a report detailing all decisions made in the previous week at the visa office London is published. Please use your unique online application number to search these reports for your decision.