I used to trim cases often till I read what Varmint Al had to say. I suggest you read his comments and decide what you would like to do:
Varmint Al's Handloading/Reloading Page
I made my own chamber length plugs based on his supplied dimensions but you can buy them, as the last poster stated, from Sinclair.
Sinclair Chamber Length Gage | Sinclair Intl
I now have measurements for every rifle's chamber length. I like to keep the brass .010" shorter than that dimension.
Factory chambers are usually very long. Some of the brass will never require trimming. If the case got to the end of one of those chambers you would have a case head separation before you needed to trim. I have seen a few chambers that were over .070" longer than new brass!