It depends a lot on the condition of the property and how old everything is. But on a duplex
1) Capex is probably the main variable. What is the condition of the roofs, foundation, driveway, paint, and other big items that need to replaced soon? If the roof needs to be replaced in say 5 years, you may even want to factor things like that on a per month basis to arrive at your capex calculation.
2) Water, sewer, garbage: Depends if there's landscaping and washers. Probably around $200 a month (my area)
3) Gardener: $50 - $100. Depends on the garden
4) Supplies: $50
5) Rodents, plumber, something gets broken: $200
6) Vacancy. It depends on how stable that tenant is. If they've lived there for 5 or 10 years, they're not very likely to move in the next few years. If they just moved in and rents are higher than market, the tenants will be more likely to move. However I might allow for 1 month vacancy every 2 years $1500/24 = $62.50
Based on that you might be looking at close to $600 + capex + mortgage + insurance + taxes. You also have to pay for your own gas and electricity, and spend more to fix up your place.