there's 2 options both requiring 1 change:
DLR to Bank and Waterloo & City to Waterloo - I think you can do this using escalators all the way.
DLR to Canning Town and Jubilee Line to Waterloo
I suspect Jubilee Line route will be a tad quicker as Waterloo and City is a bit of a faff to get to at Bank.
Suspect both routes will take you around (or just over in case of Bank) 30 mins.