There is no set length overall, but where a time is given, two hours is fairly typical. However, this can vary a lot; many authors, especially if they are less well known, will just stay for as long as there are still people queuing up to have their books signed. It is also very common for a signing to be arranged more generally for just the morning or afternoon of the fixed day, so although there may be a fixed time to start there is often no fixed time to finish.

On the other hand, very well known or busy authors may limit the time in advance (if so, people should be informed either through the venue's website or on a notice at the venue). Or a performer (speaker, standup comic etc) who is doing a book signing after the performance may have to limit the signing time for practical reasons.

You can find out more about McFly's next signing here.