At home, I check the collimation every third or forth session. I'm not obsessive about it, but I wouldn't let it go past four - you will notice the difference. But this also depends on your scope. All my Newtonians use heavy/strong compression springs and not locking screws. This means I need to check the collimation a little more readily. If my scopes are subjected to any jarring or knocks, I just check it - just takes a minute.
Another thing, the collimation process is not a tedious one. It really isn't. Once you've got the hang of it, and understand the process, you will find yourself
insisting on doing it, knowing that the minute or two that it takes makes a huge difference in the image quality you will get.
How tight a circle grouping should you aim for when tweaking the collimator - as bloody good as you can get it! While the frequency of collimating the scope I'm not obsessive about, making sure the collimator is spot on I am. Yeah it's a pain in the backside, but its just the thing to do when the full Moon is up, or when it's raining, or when you should be mowing the lawn, or cleaning out the car, etc. These are those little things that we amateurs can actually do to get our gear working to their optimum. The results are really worth it. Honest!
