How long should the retention period be when selling a rent roll?

Naturally the vendor will want shortest time and the lowest amount and the purchaser will want the longest time and the highest amount.  So therefore what is reasonable?

I am suggesting that an amount of 10-15% of the purchaser price to be held in Trust as the retention sum for a period of up to and no more than 6 months. In most of the transaction that have been negotiated, the retention period has been 3 or 4 months.

The interest received on the retained amount should be divided between vendor and purchaser.