Longtime rhymes with:
springtime, airtime, alltime, anytime, bedtime, centime, christmastime, daytime, dinnertime, downtime, everytime, flextime, fulltime, funtime, halftime, lifetime, lunchtime, maritime, mealtime, meantime, movietime, nighttime, noontime, overtime, parttime, pastime, peacetime, primetime, quicktime, ragtime, schooltime, showtime, sometime, springtime, summertime, wartime, wintertime
Longtime sounds like:
lamston, langdon, langston, langton, lansdown, lansdowne, lanston, lengthen, lenington, limestone, loewenstein, longden, longtin, longton, lowenstein, lumsden, lunchtime