Riflemen,
Case annealing 308 Winchester, once-fired Lapua brass, …
I shoot 44.0 gr. Varget under a 175 gr. Sierra TMK.
I’m full-length resizing in Redding match dies.  I realize this sets the shoulder back further than absolutely necessary, but I shoot several different 308 Win rifles and I need my handloads to be usable / chamber in any of my rifles.  
I have the $100 Anneal Rite (very manual) case annealing system using x2 propane torches.  I use Tempilaq 750F liquid detection fluid.
Question: How often do I really need to case anneal to avoid cracked necks?  Every 5 shots?  Every 10 shots?
I ask, because case annealing is a LOT of extra work, and time.  I’m trying to avoid buying the motorized carousel annealer.  - CS

For my blasting ammo, I will anneal every 2nd or 3rd reload..On my precision stuff, and any protection/defensive type ammo, it gets annealed at least once every firing, some get annealed twice per firing, just depends on what I see/feel when I size  it....

I've heard on lapua brass do it in the fifth sizing.

I have some Federal cases (FC 308 Win) (that I have never annealed) that have 10 or more firings on them.  I have retired several cases in that group for loose primer pockets and incipient case head separation but I have never had a cracked neck.  If you are pushing the shoulder back more than optimal for 1 rifle (to get multiple rifle compatibility), I would think that ICHS would be a bigger problem than cracked necks...

NOTE: I use a resizing die with a neck bushing chosen to NOT OVER-SIZE the case necks.  The expander ball has been removed from my sizing dies.

YMMV...

I anneal every time. It's not that difficult.

I also anneal every reloading cycle.