I dont currently culture amphipods, I did it a few years back mostly to make sure I had enough live food on hand for some SHs that I was trying to teach to eat frozen food, so I did not optimize my techniques. I did take reasonably good notes on my setup and maintenace though, sometimes it pays being obsessive and being way into this!!!
I used a 10 gallon tank at room temperature (75 fairly constant) that I lit with a 70w MH on a 12 on 12 off cycle (I tend to use MH for everything, Im sure less would work), the tank had 1/2 inch of sand and 2 lbs of LR rubble the only circulation was provided by a luft air pump without an airstone. I added 4 handfulls of ulva (which I had in my most productive fuge at the time), and innoculated with all the amphipods I could catch in my other systems (I dont know how many this was probably 1-200). I fed the tank daily with one pinch of new life spectrum food I had ground in a spice grinder, and started harvesting four weeks leter, there were more amphipods in the tank than I could count. I harvested 10-20 daily for the duration of the culture (5 months) and did not observe a visible change in the culture population.
For maintenance, I performed a biweekly 50% WC (filtered through a section of panty hose to save as many amphipods as possible) with water from the sump of my large system, detritus included. I also removed bristle worms I ran across during the WC, or noticed any other time.
Once set up this was a very simple thing to maintain and had the production that I was looking for. I would think that if you were just trying to restock a DT with them, you could set up the culture, keep it going for 6-8 weeks (or so) and then add all of the contents to your tank before starting it again. Also, tt takes about four weeks into their life cycle before they start producing young, which are live (the Plankton Culture Manual says this happens when they molt), which means that it would take a few months of unhindered growth before you hit the exponential growth phase (useful if you are trying to culture them as a food source).
Hope this helps!