In a wired lan, 100% packet throughput is expected. Wireless is a contention-based medium, so a certain percentage of packets will not get through the first time. The more packets and users, and that percentage can go up. What is the percentage and how much does it increase? You have to baseline your network when you have no complaints and then compare it to a time when you do....
To me 70% drops is unusable, 50% is bad, but that is just what I think....