BRTimes puts it at 8 minutes.
That is just the walking time. Journey Planners add up the station A time, the walking time and the station B time to get the total interchange time.
An example journey from NRE
1932 LOB - BHM 1952
(12 min gap)
2004 BHM walk to BMO 2012) (8 min)
(minimum 5 min gap)
2018 BMO - SOL 2026

Thanks alot everyone....25 mins it is then "officially"...I'm going to be attempting a 13 min connection time in June...which while unofficial, I think is do-able...as long as the train running on time gods are with me!
Since when has the official minimum connection time at Birmingham New Street been 12 minutes?
13 minutes is definitely pushing it given the relocation of the station entrance from the Moor Street end to Hill Street/Stephenson Street.
It's possibly a fudge to make certain connections "work". Certainly showing as 12 in the current eNRT. There's a similarly dubious 1 minute minimum connection at Gerrards Cross and High Wycombe over in Chiltern land.
The way to prevent both the 0 mins fudge and the incorrect change of train, would be to enter them as one Luton to St Albans via Sutton service, but that would cause a lot of confusion when looking at live departures online. Stations would show nominal destinations to prevent people going the wrong way.
13 minutes is definitely pushing it given the relocation of the station entrance from the Moor Street end to Hill Street/Stephenson Street.
So a bit like Wirral Line services that are entered with the origin and destination as the same station via Liverpool Lime Street, but are advertised with a destination of Liverpool Central (changing back to the origin after James Street).