The longest gap between a movie and its sequel

TVTropes unimaginatively calls this Sequel Gap, and has a fairly extensive list.

The longest gap they have with actors in common in the Live-Action Film section are the Brazilian films Esta Noite Encarnarei no Teu Cadáver (1967) (a.k.a. This Night I'll Possess Your Corpse) and Encarnação do Demônio (2008) (a.k.a. Embodiment of Evil) which both star José Mojica Marins as The Gravedigger, Zé do Caixão (loosely translated to Coffin Joe) which is an astonishing 41 year gap.

Though this is debatable as it's not a direct sequel, but parts 2 and 3 of a trilogy which began with À Meia-Noite Levarei Sua Alma (1964) (a.k.a At Midnight I Will Take Your Soul).

Another shorter option from the same list is The Naked Civil Servant (1975) and its direct sequel An Englishman in New York (2009) which both star John Hurt as Quentin Crisp which is a 34 year gap.

This might be considered cheating, but there's the The Nutty Professor from 2008 which is a sequel to the 1963 comedy. It's a 45 years gap, and Jerry Lewis reprised his role as Julius Kelp in it. However, it's animated and the original was live action.

Also noteworthy is Belle toujours from 2006, the sequel to Luis Bunuel's Belle de jour from 1967 in which Michel Piccoli reprised his role as Henri Husson 39 years later.

You can find more suggestions on Wiki here.

My favorite is The Hustler (1961), where Paul Newman reprises the role of "Fast" Eddie Felson 25 years later in The Color of Money (1986).

Czech films "Sn?ženky a mach?i" (1982) and "Sn?ženky a mach?i po 25 letech" (2008), translated as "Snowdrops and Aces" and "Snowdrops and Aces, 25 years after", share a lot of the original cast. That did not help the sequel much, though.

In Mollywood (India):

Irupatham Nootandu (Twentieth Century) -1987 :: Sagar Alias Jacky - 2009 - 22 years

Mannar Mathai Speaking - 1995 :: Mannar Mathai Speaking 2 - 2014 - 19 years

Depending on your perspective, one could make a argument about the documentaries 7 Up and 56 Up, which are 49 years apart. These were the first and eighth installations of this series, though, where each subsequent sequel was only 7 years later.

For Main Steam movies: Star wars was 1977 next year Episode 7 will be released so that will be 38 year gap

Since you did not include 'Dumb and Dumberer" number 2 I will assume you mean from first movie to last in series.

I believe some Disney movies might hold this record. Bambi was over 60 years, Cinderella and Peter Pan around 50 years or so.