You purchase a duplex and your tenants have been there 10 years and paying $150 below market. The very first things most tenants expect from a new landlord is a rent increase.
The amount you increase the rents depends on some factors, e.g. the condition of the apartment and whether or not you can afford to handle the costs to get a new tenant just after you closed escrow. But, the overall practical business model should be to increase the rents as fast as you can afford and not as fast as your tenants would like.
About 5 years ago, I purchase an 11-unit apartment building (odd number of units?) and every rent was $500 below the market. It was obvious that I could not afford to raise all the rents so high that the tenants would move and I could would not have enough money to have the loss of income and remodel all the units at the same time. So, I increase the rents for 10 units by $50 (that is $6,000 more gross income every year) and I terminated the tenancy for one tenant.
I remodeled the apartment and increased the rent from $800 to $1500 per month. That is another annual gross rent increase of $8400. In 10 years the total gross rental income is now $144,000 and when you factor in the appreciated value of the property based on the Gross Multiplier rule-of-thumb my realized profit is $187,800 at the end of the first year
You have to be insane to constantly worry about what your tenants can afford, or worry what they will do if you terminate their tenancy. They are tenants. They go rent from some other landlord.
This is what I did with this 11-unit property in the past I paid $1,225,000 for the 11 units about 5 years ago. I gutted and remodeled 8 of the 11 units. I increased the rents for almost every to $1500, $1650 and only one unit was increased to $1250.
The chart, attached, was not designed specifically for the profits I want to show, but it gives you an idea regarding how increasing the rents by the end of the 2nd or 3rd year gave me a $943,000 profit and at the end of 10 years my profit is about $2.2 million.
I own several apartment buildings and some are worth more than $7 million. By understanding that rents need to be increased to maximize profits and by understanding that tenants are only tenants (they don't get a say in business decisions) I have made $millions by doing exactly what needs to be done to achieve what I do.
By using my business model and philosophies, 9 months ago I paid off the 11-unit building and I paid off a 14-unit building on the same day. Last week, I paid off a 28-unit building worth more than $7 million and in about 4 to 6 weeks I am paying off a 24-unit building worth more than $7 million.
Why (how) can I pay off all my mortgages? It is because I make business decisions based on math and business logic and my tenants' problems are not for me to worry about. I am not heartless. I will give my tenants a 60-Day Notice To Vacate and give them an extension. But, tenants think about and know every day that their tenancy is subject to rent increases and termination,

'at-will'.