Seed packs have bright pictures outside and implausibly tiny pinches of seed inside. Many flowers, left to their own devices, will produced their own seed in abundance . All you need to collect your own is to avoid the tidy minded urge to dead head everything; let the plant flower, run to seed and then dry a little.
The time that this takes varies widely from species to species . Currently I am collecting ipomia (morning glory) seeds from the plants that have clothed my ugly wire fence all summer. Sweet William seeds come true and are ready a few months after flowering. Wall flowers also produce copious seeds if you will allow the seed heads to dry and split many months after the spring flowering. I bought one seed packet of cloth of gold wall flowers and seven years later I am still enjoying the flowers every spring! The pinch of seed I collected from an honesty plant in Greece six years ago still sets thousands of seeds each year ( as well as beautiful seed heads for dried flower arrangements ) and the few deep red holly hock seeds I swiped from a garden in Switzerland has populated my French garden with gigantic plants that have flowered three years from the same stock.
Nasturtiums seeds work well for a few years but seem to loose their vigour , but wild hoary mullian shoot up in every dry place they can find,true to type to delight the first bees of the day, every morning of summer.
I love the feel of seeds in my hand; smooth or grooved; tiny or stocky; each speck of life waiting for the warmth of spring and some dark moist earth to transform themselves into roots and leaves, flowers and again seeds to keep us all dancing in the circle of life.