Unfortunately there is no easy answer to this one! I'm assuming that you are talking about an Ethernet LAN; and not Token Ring, or WAN.
The short answer is that it depends on the network itself. (bandwidth, shared/switched ethernet, etc).
Is something that makes you think the network isn't performing as it should?
Things to look out for:
Persistently high levels of colisions on hubs could be an indication of high utilisation;
The first step in answering this is would be to drawyourself a network map, including bandwidths where you can. You can then make sure you've followed the Ethernet rules (levels of daisy-chained hubs, etc), as a breach of this could cause strange performance problems.
If there are "concentration"points, at the "Centre" of the network, you might benefit from installing a Switch at that point;
If the network already contains any data Switches (Cisco/3Com/Nortel) etc. then the packet analysis type stuff is probably of limited use, as the packet analysers will only analyse the conversations that it can see. The chances are that if you already have switches, (or even manageable hubs for that matter) you could use an SNMP/RMON tool to look at individual interface utilisation. Compaq provide a freely downloadable tool that is useful for this called "CNMS". You can find it at
http://www.compaq.com/support/files/networking/software/
This works at a much higher level, more like HP Openview than a packet sniffer.
On general utilisation guidelines, Shared Ethernet (no Switches) starts to get congested at about 30% utilisation.
Conversations between Switches can theoretically go over 80% as there are only the two devices talking, but at this sort of utilisation you probably need to be looking at increasing the bandwidth anyway.
It looks as though you might have some fun sorting this out!
Good Luck!
Jason.
