The length will depend on which breviary is used. I think the Franciscans use the 4-week LOTH but with adaptations (calendar, etc) to their community. It’s fairly short office even with long readings. I can do it (recitation only) in 15 min. or so. In community, chanted or recited (I think the Franciscans don’t use Gregorian Chant but they may chant recto-tono), I would think 30 min. max, maybe 10-15 min. more on Sundays and feasts when there’s a “third nocturne” (a bit of a misnomer because in the LOTH there’s no “second nocturne”; it’s really just 3 additional canticles followed by the Gospel reading).
The Benedictine community I am associated with uses a 1-week LOTH cycle (Benedictine Schema B) which takes them about 50 minutes, chanted slowly recto-tono, on ordinary days and 1h10-1h20 on Sundays and feasts; they start the Office at 5 am. A community doing the full traditional Benedictine Office would take an hour + on regular days.
I think the Carthusians still do a split night, praying Compline at around 7 pm and rising again at 11 pm for Vigils. Their Vigils are VERY long.
A side note about the Benedictines, many communities do now eat meat except on Fridays or during Lent; at least the 3 that I am familiar with (and two of the three use Latin daily in their liturgy, and one uses the original Benedictine schema for the LOTH). The Rule (depending on which translation) says not to eat the meat of quadrupeds (so poultry or fish would be OK), and the Rule also allows flexibility for the very young, the ill and the elderly to eat meat. As many communities have a large number of elderly monks it makes economic sense to only make one meat dish. Typically they eat meat at noon, not at the evening meal. The Rule is full of flexibility and most communities have long abandoned a strict legalistic interpretation of the Rule, but have adapted it to modern times. Of course that gave rise to first the Cistercians of the Common Observance (reform) and then those of the Strict Observance (reform of the reform). The Carthusians claim “never reformed, because never deformed”! The Benedictines on the other hand claim that theirs is a more balanced approach and leads to a healthier community. Go figure, I guess it’s whichever charism works for you and I like the balanced and nuanced Benedictine approach.
As for terminology, the Benedictines I know all call matins “Vigils”. “Office of Readings” is for the Roman secular Church and any community that has adapted that Office as their LOTH. For the most part the Benedictines except for those with a heavy external apostolate, do not use the 4-week LOTH but one of the approved Benedictine schemas with, at minimum, a 2-week cycle of psalmody, and many more contemplative houses (like the one I am associated with) do a full 1-week cycle (150 psalms per week).
