I think it depends on the type of units you are renting out and your normal rate of turnover.
Large complexes for example, typically raise rent every year. In times like today, I would guess 2-3% (rents have likely increased faster than this, but you should give a discount to avoid excessive turnover (and cost).
If you have a small number of units and correspondingly less turnover (or no turnover), the math might be different. I rent out a SFH and rarely increase the rent during tenancy, especially if the tenant is low-maintenance. This is because I miss 100% of my rent if someone moves. If you have 10 units and 2 people move that’s only a 20% hit for the turnover period (plus maintenance, paingint carpet, etc).
If you think you are below market, I would go ahead and do an across the board 3% increase. Most won;t move, but if you are lucky maybe one will. Get your tenants conditioned to annual increases, just like the large complexes do. I wouldn’t try to raise to market instantaneously. But if you are lucky one or two will move upon the rate increase and you can capture some upside for that unit.
