My vocal score is pretty thick, running to 230 pages of music. The actual length of the performance could vary quite a bit. The modern vogue is to perform Handel much faster, which is supposed to be more authentic. The bigger the choir, the slower the tempi, although it is also influenced by the musical standard of the choir, as fast singing requires precision. A lot of the solos are da capo arias and these are sometimes shortened by leaving out the middle bit and just singing the first part once. Some numbers, solos or choruses, are sometimes left out altogether.
I should allow about two and a half hours, including an interval.