That's a "how long is a piece of string?" type of question :o) It's very variable, depends on the field (popularity of project), the group (everyone wants to work at prestigious institutions and with the top people), how well the post has been advertised, how much funding is on offer, time of year etc etc. Best bet is not to underestimate the extent of the opposition but to go prepared to show that you are the best of the bunch (however many that may be!). Good luck!