RSTP proposal agreement question

Hi,

In RSTP, take a single segment where two swithches are connected back to back.

SW1 --------- SW2

SW1 sends a Rapid BPDU with proposal bit set and SW2 accepts and they are now send to be in sync.

Now consider SW1 enabled with RSTP and SW2 is STP enabled.

As RSTP is backward compatable, SW1 sends a proposal for which its doesnt get an agreement it falls back to STP and the ports are transfered through blocking, listening, learning and forwarding.

My question "is there any timer through which the SW1 waits before falling back to STP or will SW1 sent multiple proposals"

Please mention the timers if any.