Under current law, you can take the exclusion every two years: https://www.irs.gov/publications/p523#en_US_2017_publink1000...

Are you referring to the over-55-once-in-a-lifetime exemption that was replaced in 1997 with the introduction of the Taxpayer Relief Act? https://en.wikipedia.org/wiki/Taxpayer_Relief_Act_of_1997