Bartholomew Johnston, Wait of Scarborough Born 1710, died 1814 - 104 years. See Pictures section
Lincoln: William Howe became a Wait in 1810 (voted in - there were 3
others being considered). As Howe was still a Wait in 1857, that's 47 years.
York: Thomas Kilvington, an 18th c York wait seems to have had a career of around 50 years.
James Watson, one of the York waits, took receipt their salaries from 1775 until 1825 so it looks like he was employed as a wait for up to 50 years. However, in 1814-5 he was unable to play because of old age, so that might affect his chances.
Leeds: 1824 On the 7th inst. in the 87th year of his age, Mr. Thomas Laycock, for upwards of 45 years one of the Waits of this borough.
Obituary in the Leeds Mercury, Sept 18th
Selby Dickinson, Wait of Lincoln, who "died in office" in spring 1857, appears to be the last salaried Wait. See the Lincoln Waits site for details.
The Waits of Leicester are said to have continued playing regularly until the early 1950's, but were not salaried after 1835.