It can anything from two weeks to a few months. On one occasion I rang to check progress and found that junior member of staff tasked with the slush pile had left. The incumbent promised to look at my synopsis quickly. On my next check they had also left. Cleanly that large reputable agency would never come back. The solution is to send your synopsis etc.. to several agents at the same time. My experience was with books and I found that a courtesy phone call before dispatch could be helpful - it could let you know if an agent was overloaded or ready to receive.