There are no special timeouts for port 80 traffic on the PIX. The standard xlate and conn timeouts apply to this traffic. These can be modified but you want to be sure you know what you are doing. Here is the link to the docs on this:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/cmdref/tz.htm#wp1026093
Problems related to connections timing out are rarely related to the PIX timers. Can you elaborate a bit more on the actual issue you are seeing?
Scott
