If you mean "service" as in the congregated meeting -- then I think that it depends.
I have been in 4 hour meetings that just seemed too short...and 20 minute meetings that just seemed too long. I guess that we have so many congregations that are so concerned about "decency and order" that they follow the same sort of schedule that has been handed down for centuries...all the way from the pagans in Rome.
There are few requirements for a meeting listed in the Word of God. The early Church did not meet in ornate buildings with stained glass windows, fancy instruments, lovely pews and gifted choirs. They met from home to home. They broke two types of bread (physical food and the Word).
But what was the purpose of their meeting? To learn? Peter said to some that by that time that they should be teachers. To sing? We worship always. To pray? We pray always. To fellowship with others who share similar doctrinal views? But there is only one Gospel. So why are we encouraged to meet together -- and all the more as we see that day approaching (Hebrews 10:25)? "...but [u]exhorting one another[/u]: and so much the more, as ye see the day approaching." One of the great purposes for meeting together as the days grow dark, I believe, is to encourage one another. That can take minutes...or hours. Yet we can encourage one another outside of a building...or home...or cathedral. We can exhort one another here at SermonIndex. But in my opinion, there is holy intimacy when we exhort one another in person.
I have always lamented the fact that it is difficult for the Body of Christ to truly be in one accord. We are so aware of those around us. We don't pray or cry aloud when we feel urgency for fear of offending others. We don't share what is on our hearts for fear of disturbing "the order of the service." Most of the time, we attend to hear what the pastor (or guest speaker) has to say -- even when they often have little to say (but say it anyway).
I thought about something the other day. There are messages here on SI that I can hear 100 times -- and feel encouraged...and challenged...and broken...and impassioned...every single time. Yet I have sat in congregations where I didn't get anything (as far as revelation or spiritual challenge) for over 100 services! Sure, the pastor "preached." Sure, the congregation sang and gave offerings. But people arrive, sit through the meeting, go through the motions, pray at the end, and then leave. Where is the "exhortation" or "encouragement" of ONE ANOTHER as we see that day approaching? From songs? From an impersonal sermon? From the feeling of sitting in a "holy place?"
I guess I should try to give my 2 cents to your question. How long is the right time?
We should meet with one another until we have attempted to encourage and exhort -- or have received such encouragement and exhortation. If we can't meet in a public place (building, home, campground, or whatever) then we should meet and encourage one another wherever we can. Some of the most meaningful "meetings" that I have been in were in caves, log cabins, campgrounds, home fellowships, and even my college dorm! It wasn't that I couldn't pray, sing, worship, or hear the Word of God any differently. I do those things when I am alone. It is just that there was a real type of encouragement with one another. Sometimes, it was simple fellowship with just one other brother in my dorm. Sometimes, it was just speaking with friends in a meeting as we spoke about the things of God.
So how much time is the proper amount of time? As much as it takes. I just pray that we would stop trying to "force" a move of God (which probably would not be a real move of God since it is forced) by trying to make things happen. I pray that we also wouldn't try to hurry up either. We should not feel bound. We have liberty. We should have the liberty to trust Him and walk with the freedom that He gives to encourage and be encouraged. We should not fear leaving early and being judged as "less spiritual" simply because you have things to do or need your rest.
:-)
