I'd rather have at least 3 hours.. As it is recommended by Domestic-International transfer by most airlines..

Here's how it goes if you're using a 1 1/2 hour interval..

11.40 arrival
11.45 baggage claims (if you have a direct-connection flight, don't worry about it)
11.45-12.00-12.15 baggage claim (depends on how long the luggage comes out, but let's take the worst estimate)
12.15-12.30 walk to TBIT
12.30 Check in (a little pushing it, don't you think? I don't know what's SQ's Check in policy is..)
12.30-13.00 security check (depends on how long) (the gates might be closed already at this time)(if you run to SQ's gate, you'll see some unhappy faces in the GA's face)
13.10 your flight departs

So it's basically the simple run-through with my own estimations..