How to set squid proxy server download limit ?

Add the following line in the squid.conf file
Use based download limit is tested only on squid-3.0.STABLE20-2 and older versions.

acl Group1 proxy_auth user1 user2
acl Group2 proxy_auth user3 user4
reply_body_max_size 20480 KB Group2
reply_body_max_size 10240 KB Group1
reply_body_max_size 5120 KB all

For version squid-3.1 and higher , you can use IP based download limit 

acl Group1 src 10.5.0.1-10.5.0.10/32

acl Group2 src 10.5.2.1-10.5.2.10/32

reply_body_max_size 20480 KB Group2
reply_body_max_size 10240 KB Group1
reply_body_max_size 5120 KB all 

Time Based Download Limit

acl WorkingHours time 08:00-17:00
reply_body_max_size 10240 KB WorkingHours