Usually thought to be good for about 3 years under good conditions...if you're asking about ornamental portulaca grandiflora. Weedy Portulaca oleracea/purslane, probably through infinity and beyond ;)

Thanks so much morz8! I did a long search but came up empty....... Yes, I'm asking about ornamental portulaca grandiflora...... I started some seeds from 2014 indoors recently & germination wasn't good...... Started more fresh, so we'll see if that makes a difference....