I generally make sure I finish every game I buy (as long as i'm enjoying it, that is) before I start playing another SP game. The caveat is if a MP game grabs my attention, I could put the SP game down for months.
The exception atm being AC: Brotherhood, haven't even started the SP yet, been too busy Witchering.
The exception atm being AC: Brotherhood, haven't even started the SP yet, been too busy Witchering.