It really depends on how well you know Birmingham city centre, and your walking place- for example I'm in my early 30's, walk pretty quickly and know town well. I can walk from New Street Stataion to EmilyBlonde20's place just off broad street in 10 minutes. If you didn't know where you were going, it'd take substantially longer.
google maps sends you on a mission if you just type "new street station" and "broad street in" but for reference:
Come out of New St at the Boots end, and walk to your left for about a minute, and you're in front of the mailbox.
Walk into the car park next to MalMaison (there's a dedicated footpath), and take the lift up to the 3rd floor.
Walk through the main restaurant level, out past the bars across the metal bridge.
There's a small brick exit just after the ramp down to the other side of the canal- walk
Out of that and you're on broad street.
If it makes you leave the station towards the bullring, you're going in the wrong direction for broad street, and it'll make you walk in a big semi circle.