Recommendations needed for UPS

I need to have an UPS to keep up the following devices

* DLink ADSL modem
* Linksys Wireless Router

I don't need this UPS to power/backup PCs (desktops).

* What is the capacity of UPS required ?
* How much time will it be able to power on the devices ?
* Any recos for specific makes ?
* Any recos for places to buy from (in Chennai) ?