For a BOINC gypsy like me, that's impossible to answer. I rarely spend more than a week crunching a single project on my 'main' machines. For old flaky machines I tend to go for projects granting 'decent' credit or it's not worth the electricity keeping them running.
I'm happy with the credit from most projects.
There's a few I won't crunch due to stupidly low credit (Correlizer, SAT, Rosetta, EON, YAFU, Slinca, Mindmodelling, WEP-M2 to name the main offenders).
Others I won't crunch because of long WUs with no checkpointing. And some I won't crunch because they seem to be a complete waste of electricity. And some I won't crunch because they need a version of BOINC that sucks (pretty much anything after 5.10.45). And some I won't crunch because they only run on Linux. And some I won't crunch because the admins are d*cks
Funnily enough, 3 the 4 big-hitters for CPU work are down or out of work. I smell a conspiracy
Anywoo, that leaves me with 41 projects on my list to choose from that offer enough credit to keep me happy. I remember when there were only 6 project in total to choose from
End of waffle
Al.