how many coins are reserved for the faucet and how long will it last?
Indefinate

so where do the coins come from?

Spencer Lievens and Steven Saxton both mine on the pool and provide the coins to the faucet.
With our mining, the Sterlingcoin faucet will never run dry.

Here you see my miners working away.

Stop by
http://faucet.sterlingcoin.org.uk and you will win a random amount from 0.00083400 SLG 0.04167000 SLG.
You can return every hour if you wish! More details can be found here:
http://faucet.sterlingcoin.org.uk/doc/SterlingcoinOfficialFaucet.pdfEvery 24 hours (at about 0 GMT) your accumulated total will be sent to you.

All courtesy of the Sterlingcoin developer team.