If you set up a program carefully, most maintenance will be routine.  Emergency maintenance then will be minimal.  There is a third category: deferred maintenance.  This takes into account items that will need care sooner or later but may not require immediate attention.

You must keep a sharp eye out for deferred maintenance items, because they have a tendency to build up and then cause sudden, serious breakdowns.  For example, if six water heaters fail in one year, you can expect that the other 22 installed @ the same time will fail shortly.  Be prepared for this eventuality and have enough money to pay for replacements.

Equally important, deferred maintenance may be noticed by prospects who will downgrade the property accordingly. 

Most apartment complexes have long lists of deferred maintenance items, such as cracked concrete sidewalks or curbing, dead shrubs, worn patches in the roadway or parking lot, and clogged water heaters.  Some of these items, such as the dead shrubs and the holes in the pavement, should have been corrected by routine maintenance.

The purpose of planned maintenance is to avoid financial surprises.  You do this by constantly allocating money for necessary repairs and rotating the replacement of aging equipment and components.

Practically every component of and piece of equipment in an apartment building has a predictable lifespan.