Overnight to a week?
 

I have a schwinn sierra 700 and already have rear panniers . Would this bike work for overnighters to a week long tour?

Your bike is fine, as long as you can ride it all day.

ok, It has a steel frame . How can I find out how much weight it will handle?

Just make sure it's mechanically sound, wheels are true, and the tires are in good condition. You might want to add some bar end extensions for more hand positions. The gearing is right for loaded touring in hilly areas.

As to how much weight it'll handle, I doubt you really want to test that as it would involve breaking something.

With decent racks front and rear, it'll for sure haul at least 60 pounds of gear safely. That's about 20 more than you'll need, unless you'll be crossing a desert and need 2 or 3 gallons of water. Unless, of course, you weigh like 275 pounds or more. Then there might be a problem.

Add weight and ride?

It will take more weight than you should put on it;) Realistically, you are going to be limited by the racks you use for carrying the weight not by the bike. Your bike will handle better if you put front bags on it and carry 60% of the weight low on the front wheel.

I still have an older Sierra 700 that served me well on many
smaller tours before I was sure that touring was something
that I really enjoyed. I still have the frame sitting in the corner
of my storage shed. One of these days I plan to rebuild it when
I have enough spare parts. It had great gearing and the wheels
held up remarkably well over the few years I toured with it.

It will serve you well. Would suggest bar ends for the added hand
positions and comfort.

Kayakdiver

The bike should be fine. Go on an overnighter or two. Work up to a week. After a few trips you'll likely start to develop an idea of whether you're satisfied or want to upgrade - how to carry your stuff, etc. I think questions are best answered by experimenting.

Or else ask a question here and do EXACTLY what the first responder tells you. ;)

Or trekking bars, although that's a slightly bigger investment.

OP, the bike should be fine. Ride and have fun!