It kinda depends. I have one duplex that has been one thing after the other. Roof, hot water tank, furnace, leaky toilets. Others that haven't needed anything.
My suggestion is to figure out the age of your major stuff, and conservatively estimate its useful life. Set aside something each month to cover for it when it does need repair or replacing.
Hot water tank: 10-15 yrs max life, $650 if installing yourself, otherwise $800
Furnace: also 10-15 yrs max life, $1500-$2000
Roof: $7000-$17000+, depending on size and condition. 20-30 yrs max life. Those are my ranges for recent full tear-offs with deck replacement
You'll get lucky with some stuff in that it'll last much longer than normal. I have a hot water tank from 1979 that is still going strong, I don't even look at that thing for fear it will crap out on me. I did a re-shingle on a duplex in 2005 and the roof completely failed last year. Full tear off, ouch.
Roofs are expensive and can really ruin your day when they fail so make sure you have a plan for that. Have someone come out and give you an estimate if you're not sure -- they can tell you how much longer you have until replacement, and can give you a dollar figure to go on.
I don't really budget for maintenance per se, that just comes out of my cash flow as I keep after things or if repairs come up.