How far below market do you keep a good long term tenant's rent?

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I have kept rent very low on one of my units because it needed work. I'm getting work done now and would like to bring it closer to market. I'd love feedback on the following:

Tenant has been great for 15 years. His rent is now $270 below what nearly identical units are paying. In this area, same square footage, amenities, etc. is upward of $1900/month.

Is $270 too big a gap? If so, how much would you raise? Would you increase one increment, or over time?