It varies in each parish. Part of the time people will just take turns spending time in silent adoration before the Blessed Sacrament.
In our parish we have a service each evening of 40 Hours with Evening Prayer from Liturgy of the Hours, a Scripture reading and homily with a guest preacher (kind of like a parish mission), and Benediction.
Our bishop requires each parish in our diocese to have 40 Hours once a year. Each church does it a little differently, but it’s a wonderful practice.