I'll add my experience. Moving from landline to Google Voice took 9 days from start to finish. The main issue I had was with Google Wallet.
I ported my landline to T-Mobile on the afternoon of Thursday, October 11th. I could dial out on the T-Mobile phone starting the morning of Saturday the 13th, but phone calls in still went to the landline. The service stayed "dual-homed" like this until Tuesday the 16th. T-Mobile support was sympathetic but said that they were waiting on my landline carrier to complete the port.
Tuesday night I created a new Google account to use with my phone, and set up Google Wallet. I then walked through the porting procedure in Google Voice. When I went to pay with Google Wallet, I got the following error:
Uh oh. There was a problem. We couldn't complete your purchase because of a technical issue.
I tried several more times over the next day but had the same problem. Wednesday night I called Google Wallet support. I was told that the account was under review and that I should be able to use the Google Wallet account after 24 hours. However, by Friday night the account was still under review, and the review was not expected to be completed until after the weekend.
What I eventually learned is that because I had previously used the same credit card on a different Google account that the account had been flagged for review. I was told by multiple Google Wallet support staff that the account was under review, not the card; adding a different credit card to the Google Wallet account would not expedite the process. Google Wallet support gave me conflicting information on how long the review should take, varying from a few hours, to 24 hours, to 72 hours, to an unknown length of time.
Fortunately, I had another credit card that I could use, and so Friday night I abandoned the Google account that I had planned to use and created a new Google account. I set up Google Wallet on this account using my second credit card. I was able to pay for the number port to this new account, and within 24 hours (the afternoon of October 20) the port was complete.
tl;dr: Set up your Google Wallet using a credit card that has never been used with another Google Wallet account, or be prepared to wait at least 72 hours for the card to be able to be used with the new account to pay for the port.