That's a rather subjective question and entirely depends on
a) what you're doing over the switch. Wether it's interactive or streaming, or bulk data transfers.
b) What sortof switch is it. I'm guessing as a 100MB it's a 'store & forward' type of switch, but I could be wrong.
c) How are you doing your measurements?
Depending on any of the above, I'd say around about 80% of capacity is when you start seeing degraded performance, but this isn't a T1 or something where your saturation will hinder the passage of 'ACK' packets for instance and the whole thing will crawl. You could conceivably reach 100% saturation of 100Mb port, and if the backplane on the switch is sufficiently capable see no speed decrease in traffic at either end.
So um... Why not tell us what you're seeing and then we can maybe clue up some more.