Lettuce will usually stay in a harvestable (tasty) state for around 3 weeks after maturing (at least in zone 6 for April-May lettuce plantings this is the case for me). If you are in zone 5 I imagine it will be about the same. You may want to plant on 3 week intervals. I start my plants indoors then transplant into the garden. Even though it is just me and my husband as well, we plant quite a lot of lettuce and harvest more or less depending on the size of the heads. I would say we harvest 3-4 heads per week once the plants are mature. This is using varieties like buttercrunch (makes a nice compact head), romaine, and oakleaf lettuces. So you should definitely plant a bit extra (worst case you can give a bit away and make some friends). We use row covers to start transplanting early and the lettuces love it. We usually have to stop growing it by July as it is too hot and the plants just bolt to seed. To still have greens at this time, we plant swiss chard as well in the garden around April or May. This biennial green produces nicely right through the summer and can be used like spinach in salads. Later in the season (September) we go back to the row cover method and grow lettuce right into November (even December if the winter is mild).

I am (finally) planting my lettuce transplants this weekend and I am very excited!