I'd review monthly on anything with variable expenses (vacancies, older systems, insurance renewals) and do a deeper quarterly review by door. Tax time is too late to catch a unit that's been quietly underwater for six months.
What I track per property, same format every month:
1. Collected rent vs scheduled rent (vacancy and concessions called out)
2. PITI plus actual insurance/tax escrow changes
3. Repairs vs a fixed % reserve (I don't treat CapEx as profit)
4. Cash left after reserves, the only "making money" number I trust
Rent-in is vanity. Cash after reserves is the scoreboard. If you only look annually, the loudest property usually hides the worst one.