Since you said customers I'm assuming this is a business. You didn't mention wired or wifi, so can I assume wifi?
I know this link is to a product in the UK and you might not live there but at least it's something that will get you pointed in the right direction http://www.broadbandbuyer.co.uk/Shop/ShopD...?ProductID=3110
Linksys routers do have something that you could try to use but it's very limited http://www.cisco.com/en/US/products/ps1002...080a35706.shtml
The worse part is that it's limited to 10 configurations.
I found one article where Netgear has something where you can not only limit the time (sort of) but also what type of sites can be visited, the problem there is that an extra piece of software has to be installed on both the main computer and the client computer as well. No customer is going to go for having to install something just to use the wifi.