Hi Andreas,
I have a 5'' reflector and when it is collimated it can stay like that for about half a year or so.i see you have a 6'' so once collimated you wont have to worry for a while.
if you got a collimating cap use it, all i use is the dust cap with a tiny pin hole in it it works great.
If you want to get accurate results then get a chesire or better yet a laser collimater as visual collimation can be inacurate.
The bigger the scope the more you have to collimate it .
Orestis
happy viewing
