Like meriad, I commute to London daily to work, and have 2 small children so weekends are all we've had. My wife then started night shifts so with the Sat morning lie in that now leaves us short Sat afternoons (with short winter days and poor weather) and Sundays if there arent other plans.
We bust a gut over summer and got a LOT of work done and during the hot spell I cycled up in the evenings to do the watering etc, but now we're taking it a bit easier as there simply arent enough hours in a week! This weekend we're forecast heavy rain so not much happening allotmentwise...
I'd suggest to anyone taking on a plot with busy lives to not rush things like we did, as long as you do 'something' regularly you won't lose your plot. There's always something you can grow so just work a bit at a time, clear and plant, clear and plant. If it takes 2 or 3 years to get fully up to speed so be it, just make sure you do at least half a day a week and keep those weeds down!