Assuming you're using USB 2.0 (3.0 is becoming more and more common) the official limit on cable length is 5 meters. Doesn't matter if it's an extension cable, as long as the connections are good. Since you're right at the official limit, you probably want a powered hubpowered hub.
That's assuming all your devices are up to spec. Low-data devices like your keyboard and mouse are probably OK, but I own an older USB printer and hard disk that just don't work through a hub. I imagine newer devices are better made, but reading the docs and running a few tests might be in order.
Incidentally, if using USB 3.0 is an option, that pretty much eliminates distance issues. In that spec, 5 meters is not a firm limit, just the maximum past which data transfers are not guaranteed to run at the spec's humongous full speed.