By this point you have talked back and forth twice and can measure how well you are getting along.
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.
This reason is acceptable, especially considering the hassle that some women end up going through when dating online.
In general, my rule is an invitation to meet should come by the third email you send.
I once talked to a woman for two months before we met.She was very nice and we got along well so I was willing to communicate but let her know I’d be talking to others while she gained comfort.