I have to say I run lots of calculations frequently...several times a week, daily even, how long to pay off the mortgage with what repayment, different housing, FI scenarios, and check the balance of my stache etc.
However the end of the month I chart the figures. I never know what the bank will charge in interest each month until it happens (although I guess I could calculate that too, but I'm not that big a maths nerd). Its arbitrary, but the monthly day of the charting of the figures provides me with delicious anticipation. Now I have 9 months of figures seeing progress is much more fun.