Unopened bags of paint will keep far longer than you will need. As has already been stated, the key points are
Don't open the sealed bags
Keep at room temperature, out of direct sunlight.
Rotate or shake bags once a week or so.
Here's the key point. If you buy sh*te paint at the start, it's always going to be shi*te. Spend an extra £5 per box and buy good stuff, you won't regret it.