Roth Conversion

Years ago I left a job and rolled over my 401K to a traditional IRA (or rollover IRA).  Now I think I want to convert the traditional IRA to a Roth IRA.   I realize the funds I convert will be treated as income and taxed as income.    Are there limits to how much I can convert per year?   Can I do a series of conversions over several years to reduce my taxes?