Hello Ajay.
In order to compare the convergence times of STP and RSTP, we must see what happens when a change occurs on a link. Here is how they differ. We always assume default values for timers:
STP
When a link goes down, STP waits for the Max Age time in order to consider a link down. The Max age is 10 times the hello timer and the time between hellos is 2 seconds. So it will take 20 seconds to consider a link down. At this point, the root bridge is reevaluated. If the original root bridge still has a connection to the network, it will receive Hello BPDUs and nothing will change. Otherwise a new root is elected. This can take several seconds.
Once this is done, the ports must be reevaluated as well. Root ports and designated ports are then chosen.
After the port roles are chosen, the root port will transition from Blocking to Forwarding. In order for this to happen, there are two stages in between (Listening and Learning). Listening has a duration of 15 seconds. During this time, no data packets are sent or received. Once this is done, the port transitions to the Learning (another 15 seconds) and then the Forwarding state. Assuming a new root and new root ports are selected, a single port may be inaccessible for at least 20+15+15 = 50 seconds during STP convergence.
RSTP
RSTP is faster. Thus the term Rapid STP. RSTP waits for 3 times the the Hello timer for a total of 6 seconds before a link is considered down. When a root port fails on a switch and a new root bridge has to be elected, RSTP momentarily blocks all its ports eliminating loops and thus eliminating the need for the Listening state, so we save another 15 seconds there. And the Learning stage is quick because the switch sends an RSTP proposal message immediately and receives one right away. So the port can go into forwarding state immediately. So under similar circumstances, RSTP can shorten the convergence time from 50 seconds to about 6-10 seconds.
Keep in mind that different topologies will have different results depending on which links go down. The above description should be used as a guideline, but it gives us very good insight into the differences one can expect with the use of the STP and RSTP.
I hope this has been helpful!
Laz