Generally speaking, in various campaigns which level do you start to encounter enemy Spell Resistance. Should one start to find means of increasing his caster level around level 8? 10? 12?
I can't remember the name off the top of my head, but there is at least one creature that could reasonably be fought at level 1 with SR. In my experience, it doesn't start becoming a more serious concern until around mid levels. To me, that means around level 8. I believe there is a dramatic increase in occurrences of SR after that as the "real deal" outsiders, abberations, and fey become more CR appropriate (not an exhaustive list of creature types with SR, btw).
There are plenty of creatures down all the way to CR 1/3 that can have spell resistance, it's just not as common at low levels. Similarly there are plenty of high-CR creatures that don't have it. SR does become more common at higher levels but it's not a hard cutoff, it just gradually becomes more common. Exactly when it becomes a big deal depends more on your specific adventure than your level.
I think 8-12 is about the range where you should be getting more concerned about it, but without knowing your adventure ahead of time it's impossible to know when exactly it's going to become a big deal. Ordinarily if you aren't preparing for SR you want to have spells available that aren't going to be impacted by it.