A frame rusting depends on the weather its exposed to. If no water is present than it will last basically forever. If its left outside often or ridden in the rain often enough then it will rust. The more water the faster. The more salt water the faster. Also, where the rust is will determine if it crumples. The top tube doesn't carry as much force as the down tube and seat tube or the chainstays.
You're asking a totally unquantifiable question, thats like asking "how long will my chain last" there's too many variables.
Also its STEEL not steAl.