Ah, you should have phrased it like that in your original post.

The answer is: there is no specific time frame. Here's how it works:
You have a duty to take reasonable care of the goods whilst they remain in your possession. You will be liable for any damage caused to them whilst they are in your possession.
Before you can keep or sell the goods, you must serve
two notices on the seller to collect the goods (basically you write two identical letters and post them by recorded delivery - very important. For evidence purposes, I recommend you also keep a copy for yourself).
-The notices must state that the seller is responsible for collecting the goods.
-You must also state details of where the goods are kept, including your name and address.
-You must give the seller a reasonable amount of time to collect the goods, e.g. 3 months (state this time frame in the notices).
-State that you intend to sell the goods if they are not collected within the time frame given.
-You may also state reasonable storage charges, if any.
If the seller has not collected the goods after lapse of the time frame given in the notices, you are legally entitled to keep the goods or sell them.