By default, in Corona there is no limit set for its progressive rendering and you can stop it at any time you want. However, there are currently three ways to limit your rendering:
1. Enable Time limit for rendering:
go to Render Settings > Corona > General Settings > Progressive rendering limits, set Time limit other than 0
2. Enable Pass limit for rendering:
go to Render Settings > Corona > General Settings > Progressive rendering limits, set Pass limit other than 0
3. Enable Noise level limit for rendering:
go to Render Settings > Corona > General Settings > Progressive rendering limits, set Noise level limit other than 0
Note: You can use any number of limits at a time, or each one of them separately. Rendering will be stopped as soon as any of the limits is reached.
See: How long should I render my image? , How many passes is enough?