There's no exact answer, since much depends on growing conditions. In general, if you start them in a greenhouse or a warm room under a growlight, plan to sow seed 8-10 weeks before the last spring frost (occurs around April 10). You'll need to transplant them at least once before they go into a flowerbed or planter. They should bloom within 10 weeks of transplant - so, to answer your question, they should bloom within 20 weeks of sowing.
Primula seed may need prechilling before sowing. Ifyour seed packet/catalog/source doesn't confirm that your seed is prechilled, mix seed with moist seedstarting mix, and store in a plastic bag in the fridge for 3 weeks. This simulates winter and breaks seed dormancy. Hope this info is helpful!
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