Can i shorten my time machine backups?

TM should automatically purge oldest backups when it need space:

Back up your Mac with Time Machine - Apple Support

Note this part:

If, after the first backup, incremental backups seem too slow, I would suspect your external drive and/or its cabling. For a modern iMac, the drive enclosure should be minimum USB3. If you are using an older USB2 drive for backups, the time required for anything to happen with TM will increase by a factor of 10.

Also make sure Time Machine is not backing up other attached backup drives. It might try. You can exclude items from being backed up using the "Options..." button in the bottom right of the TM System Prefs pane.

No you cannot shorten them, please let Time Machine run as it was designed to. When you try to modify it, almost always this results in a disaster. Time Machine will automatically make space when it senses the Time Machine drive is filling up, please study how Time Machine works and you will see that. Also, when buy an External Hard Disk for Time Machine it is wise to buy one 2-3 x the amount of data it will be backing up.

For example, if your Mac has a 1TB drive on it however it only has 300GB stored on it you should buy an External Hard Disk that has 1TB to 2TB, these are relatively inexpensive these days. Also, it is wise to buy quality EHD's as they can prove less expensive in the long term. Many experienced users on these forums prefer the OWC Mercury Elite Pro due to their high quality, reasonable cost, ease of services it, and the service the vendor provides. In short, they have no down side. Avoiding cheap "Walmart Special" type EHDs is wise as their enclosures tend to let the drives over heat which causes the HD to fail. This typically includes the WD, Toshiba, Seagate brands.

Manually deleting Time Machine backups is a big No No. It will plug up your Mac’s Trash and bork your backups. Which will force you into erasing the backup drive and starting over.