It depends largely on the size of the target and whether it is a start-up or long term player in the market. If the target is start-up, there is probably less data to analyse and hence the DD is usually short (the protection is then achieved through relevant provisions in the asset/share purchase agreement). On the other hand, if the company is established player, then there is more info available and to be analysed. Also, the size and importance of the deal may influence the length of DD. It can range from couple of days up to several weeks. I think it is difficult to tell the exact time. The key obviously is the preparation of the buyer (what info it requires) and quality of input provided by target.
It is also to keep in mind if the target is a public company. I was recently on a transaction which the target was a public company and we had to undertake the due diligence within 2 weeks. This is due to the sensitive nature of the information that we received.