Strange my post disapeared...it really depends on the time of day, how you are getting there!
It is 15 miles from terminal to DLR and can take anywhere from 20-45 min.
Yes, shuttles are easy peasy without a reservation, what takes longer is if you are the last drop off!!! Also, they generally wait to get some other passengers, they say they will only wait 15 min, but once I sat on the van for over 30!
However, if 4 or more, I would get a taxi, same price and faster!! Maybe even 3 is same price!