As with all seeds, germination is based on soil temperature. For sunflowers, they prefer a higher soil temperature (over 50 degrees). Planting sunflower seeds into cold soil can cause seeds to go dormant and delay germination. The average spring frost probability is up to May 11, so for best results, use a soil thermometer and test soil temperature before planting. Planting may be as late as the end of May.
https://cmg.extension.colostate.edu/Gardennotes/742.pdf

On a side note, grow bags do require more watering than traditional pots, so a moisture meter may also be something to consider. Sunflowers need regular watering for both a strong root system and to help maintain the healthy stalk. Shorter varieties of sunflowers are available that will grow 12-36 inches. Gurney's, Burpee, or Park are all reputable seed companies.

Good luck with your sunflowers. For more information, there is a National Sunflower Association:
https://www.sunflowernsa.com/magazine/articles/default.aspx?ArticleID=2964