Managing Time Machine Disk Space

I have a NAS server that I use to back up all my computers (3 PCs, and 2 Macs). For various reasons that I won't bore you with, I cannot partition that storage into multiple partitions. Is there a way to control how much disk Time Machine uses before it starts dropping old backups? I do not want it to use up unecessary space to keep backup longer than needed. I there a way to control the amount of disk space available to TM or control how long to keep backups?

Thanks.