Checkpoints write modified pages from the buffer cache to disk. When you make changes, they are not written to disk immediately. Rather, they are kept in memory and checkpointing occurs periodically. You can't set a checkpoint schedule. It's an internal algorithm.

There are also events within SQL Server which will trigger a checkpoint. Things like an alter database, backup log and some others which escape my memory now.

Also, you can run a checkpoint manually, but I've never had cause to actually do this.