I guessing you are a WDW person? First trip to DLR?
Outside of the time you spend eating or waiting, it isn't going to take much more time than a sit down meal in the park. Unlike WDW where leaving and going anywhere outside of the park is a pain, DLR is basically like one big area. Once you have gone through security, you have access to DTD, DL, and CA without having to go through security again. DL and CA are separated by a plaza with about the length of a football field between them-- hopping takes 5 minutes (unless during current times you go right when hopping starts for the day). DTD is right on the side of the Plaza--- to get to the DTD area it is not more than the walk from some of the closer bus stops to the park gate at WDW. It is kind of like a T--- DTD is the base and from the base you would branch off left for DL and right for CA. One of our favorite things about DLR is everything is right there together and it takes very little time to get anywhere.
Pretty much any restaurant at DTD isn't going to be more than a 10 or 15 minute walk from the front of the parks, if that. We stay at the
Disneyland Hotel, and it usually takes us 10 minutes to walk from the front of the parks back to the hotel, and that walk is down the main path from one end of DTD to the other. So, maybe 30 minutes more than eating at a table service somewhere inside the parks. But, for example if you were back by Small World at DL and walking to Lamplight in CA ---- or back at Pixar Pier at CA and then going to Blue Bayou or Cafe Orleans at DL, it probably wouldn't be much of a time difference than walking from the front of a park to a DTD restaurant. We don't leave often because we think the CS in the parks is much better than WDW so if we don't have a TS ADR we just get CS, but if there is something you want to try in DTD, I would say do it.
By WDW terms, I would say think of it as walking from EPCOT to Beaches and Cream for a meal. The walk from DL to DLH is about the same as walking from the EPCOT International Gateway to Yacht Club.