Depends where you are. But generally you can plant up until the ground freezes. In the mid-atlantic the landscape contractors are still planting although this is towards the end of the season. Soil temperatures are averaging in the low 40s. Plant 8" deep and mulch afterwards.
Bulbs are 12/14
The mature size of these bulbs is 16 x 9 inches.
Under good growing conditions, they should outlast any of us. While some kinds of bulbs tend to dwindle and die out, daffodils should increase. Daffodils multiply, year after year you will see more blooms come out. The flowering season is from six weeks to six months, depending on where you live and the cultivars you grow. After blooming, let the daffodil plant rebuild its bulb for the next year. The leaves stay green while this is happening. When the leaves begin to yellow, then you can cut the leaves off but not before.