It won't 'go bad'...just keep it in a cool, dry place. I put 2-3 tablespoons of milk thistle seed powder in my Superfood every day. And yep, you want the powder - there's no way you could chew or digest the seeds.
And just so you know, that whole 'flax seed goes rancid right after grinding' is something that's debated amongst the "brains that be". All seeds (well anything) degrades somewhat after it's been "opened", but that doesn't necessarily equal "rancid". They say rancid flax seed smells 'skunky', but I've never smelled that smell...and I used to keep a shaker of ground flax seed on my countertop and shake it on all my salads.
Your liver is going to LOVE the milk thistle seed!!
Healthiest of blessings -