How often is the constitution amended?

Not very often. The Constitution of the United States, originally ratified in 1789, has been amended only 27 times, with the first 10 amendments, known as the Bill of Rights, enacted all at once in 1791. The 18th amendment, relating to Prohibition, was repealed by the 21st, so there have really only been 16 changes to the Constitution since the Bill of Rights. The most recent amendment, the 27th, relating to congressional compensation, was enacted in 1992. The one prior to that, the 26th, giving 18 year olds the right to vote, was enacted in 1971.