You can get several weeks worth of leaves from cut & come again lettuces and other salad plants, like spinach, chard and wild rocket. You do need to keep harvesting regularly though.
You can either pick off the larger, outer leaves and leave the centre baby leaves to grow, or simply cut across the plant to about 6-8cm which will leave the centre leaves intact.
It is better to harvest and throw excess leaves in the compost pile if you’re not going to eat them – once the plants reach maturity they will naturally produce flowers, and you want to keep them leafy for as long as possible.