On the advice of the head of the engineering department at Northern Lights/Lugger, who is a good friend who we took with us to California to check out the GB we subsequently bought, a diesel should be warmed up enough to get a decent head temperature before it's asked to do any work.
We needed to learn about diesel operation when we bought the boat, so we used him as our teacher. He knows a lot about FL120s and most other engines in addition to the engines his company makes. His advice to us with the FL120s was to start the engines when we started getting ready to leave. In other words, when we were ready do disconnect the ground power cable, turn on the electronics and set them up, single up the lines, and so on. By the time we were ready to cast off--- which for us is about ten minutes later---- the engines will be ready to work, although he said we wouldn't want to go to a high power loading until the engines were into their proper temperature range. So that is what we have done the last nine years.
While we use our boat year round, there can be periods of a month or more during the winter when the combination of my work schedule and the winds prevent us from going out. Also on the advice of our friend, and of our diesel shop, if we get to four weeks without being able to go out, we'll run the boat in its slip. We run the boat in gear, one prop in forward, one in reverse so there is little strain on the lines, and we run the engines at about 1200 rpm. This gets both engines up into their operating temperature range quite fast and we run them at temp for about 15-20 minutes. We initially did this out of gear and the engines never got up to operating temperature at all. Our diesel shop said do it in gear, and this makes the difference.