You have to check your local law, both Ohio and municipal, to see if you are under some kind of rent control. If not, generally they can increase your rent anytime and by any amount with 30 days notice.
Many parts of the country are now going through a re-adjustment of rental prices. In certain very desirable parts of Los Angeles, after 2 years of double-digit rent increases, forecasts call for an average of 12% increase this year. California enacted a new law, effective January 1, 2001, that rent increases of more than 10% require an extra 30 days notice.