NCF operates over 200 modems lines, shared by anyone registered with NCF. Generally you can use as much modem time as you like, on a first-come, first-serve basis.
If a modem pool becomes fully busy, NCF's modem sharing software gives guidance about how to share the modems fairly -- in busy times, people who have used more than two hours of "peak period" time (noon to midnight) are limited to make way for others, until the demand for modems lessens.
Here are the details:
Sessions not involving NCF modems (eg., "telnet" or browsing NCF web pages from the internet) are always as long as you like.
Sometimes modem connections break due to line noise and other problems.
You can conserve connection time and share modems with others by working offline if possible (composing email, etc), and then connect when you need network access.
NCF modems are supported by donations from NCF members. If congestion becomes a problem, NCF members decide whether to install more modems and/or shift their usage to less busy times.