A quick search on Tokyo Transfer Guide (www.tokyo-subway.net/english/index.html) shows a travel time of 32 minutes for the anti-clockwise direction and 33 minutes for the clockwise direction so I'd just get on the first Yamanote train that arrives!

All Keisei Limited Express, Skyliner and local trains stop at Nippori which is where the transfer to JR Yamanote is easiest. You go up one set of escalators, exit the turnstiles and down another, all of which is not more than a couple hundred meters max.

Hi Sammy

Thank you for the suggestion, we did the transfer at Nippori station, it was easy. We used the Keisei Limited Express train from / to aiport, easy & cheap.

Emmayee, I hope you do understand that the Keisei Limited train is not the most convenient way to reach Ebisu? A faster route would be to take Narita Express to Shinagawa (there are trains leaving at 3:45pm and at 4:15pm), then change to the JR Yamanote line where Ebisu is only 4 stops. The ride from Nippori to Ebisu is much, much longer, and, at that hour of the day, is likely to be extremely crowded.

Well apparently they used the train just fine.