Your date might remind you of your time constraint (“But I thought you needed to be home at 8…”). People who are dating online actively will appreciate your planning for a bad date and like the fact that you are saying to them “this is a good date”.
If you would rather not let them in on your planning for the worst-date-scenario, just say that you’d prefer to cancel your previous plans and spend your evening with them.
By this point you have talked back and forth twice and can measure how well you are getting along.
If you enjoy talking on the phone or prefer to do so before meeting someone, treat this step like you would the first date by asking for the phone number by the third email.
I never went for phone conversations as a stepping stone from internet communication to meeting.
I tried phone conversations early on but had a few bad experiences with “excessive calling” and one case where a girl took it as a personal offense every time I had to get off the phone.
It is personal preference on how long to wait but there are very few reasons to delay.
After all, contacting someone is just a pre-cursor to the first date.