Hello, I'm curious as to how long great festal vespers for the evening before palm sunday at a typical OCA parish may be? Thanks in advance,
Priolo

I don't know. In ACROD about an hour or so.
However, I am curious and this is as good a place to ask - not of the OP but of all of you. In your personal parish experience, what sort of crowd do you have at such Vespers - not Holy Week or even the Nativity - but in general? A percentage of your congregation and the Diocese of you parish would be helpful.
Sadly, we have a handful and as far as I can recall, that has been the case. Our Liturgy will be packed on Palm Sunday as will most of the main Holy Week services, but not Vespers on a Saturday night - even for a big feast like Flowery Sunday ( квітковий неділю, Kvetnatá Nedeľu )as we Slavs called it.
At my OCA parish, IIRC, it's about an hour.
As for attendance at "regular" Vespers, it's usually about five or six, although at times it's been just me and the priest and sometimes swelled to about a dozen. Sadly, in the past year we lost three regular attendees when they moved away, which was extra sad for me because they were my age (generally speaking..I'm a bit older but am young at heart) and we all shared similar interests.