You should be careful while using this setting
Example: If you have 2 clients and set it to distribute over 30 days, the first one will go immediately and the second will go after 15 days.
Using Distribute Clients over X days will assign a time to each client and only allows the client to get the update at that time. If the client misses it, it doesn't get the update until the cycle starts over. Example: That client that had to wait 15 days is offline on Day 15. On Day 16, the SEPM denies the update. On Day 17, the SEPM denies again... this will continue until Day 31 when the SEPM will assign new times to the clients. Even though this is an extreme example, you can see how this could make clients take a long time to get their updates. Even on a more reasonable 2-3 day distribution, a client could miss a number of consecutive update cycles.
https://www-secure.symantec.com/connect/forums/complete-behavior-install-packages-and-upgrade-schedules