uk only ,buying a repossed house ,?

Most of the time it's 28 days. However, if you do hit the deadline and you haven't completed, it doesn't necessarily mean that you loose out. As long as you are seen to be doing everything you can and the timescale looks reasonable, they will usually stick with you. Unless a buyer comes along that can complete quicker.

Generally lenders are keen for quick sales when selling repossessed houses. But there is no specific timescale that must be met. Because the house is vacant there is no "upper chain" so depending on your circumstances it is usually fairly easy to complete quickly.

Its different if you have bought at auction. There it is generally a condition that you complete within 28 days. But you need to have the finances, and pre-exchange formalities sorted before you bid, so its not usually too difficult.

Its like buying a house from the bank who legally own the property.

If you mess about then the bank may sell the house to some one else.