When we open a package of lettuce seeds and use some of the seeds, then keep the remaining seeds in their package at room temperature, does it affect their viability?
Yes, this will affect the seeds' viability but depending on your climate, it may not be enough to worry you. In general, lettuce seeds last about 2-5 years. If your climate is hot and humid, the shorter end of that period will apply. If it is cool and dry, they might last longer.
If you are in a hot, humid climate and want to extend their life, you can store them in an air-tight container in your refrigerator. If you have any silica packets on hand, you can place those in the container to help keep the humidity inside the container low. If you do not have any silica packets, you can wrap a little powdered milk or rice in a facial tissue and place this packet in the container to control the humidity.