Welcome to Four Daily Wins, your one stop reference for how to get the most bang for your buck in the Magic: The Gathering Arena Free to Play Experience.
When to Open Packs, AKA Exploiting Duplicate Protection
The Bottom Line: Don’t open packs from the current set until you decide you’re done drafting that set for the season. If you only intend to draft until you have most of the set collected, then you should hold off until you have 150 packs in your inventory (but that's a lot, so if it sounds too daunting then at least wait until you have 100). This will almost guarantee that you’ll have a playset of every rare in the set, and you can fill in anything missing with Wildcards.
If you don't plan on drafting, you can safely ignore this, and instead open all packs as soon as you receive them.
Open packs from older sets as soon as you get them.
But Why?
When you open a pack in your inventory, the rares you get are guaranteed to be cards of which you have less than 4 (unless you’ve already collected 4 of each rare in the set). This is called Duplicate Protection.
Duplicate Protection does NOT apply to rares that you draft. If you already have four Searslicer Goblins and you draft a fifth, you’re not adding to your collection (though Arena gives you 20 gems every time this happens).
This means that it makes sense to draft the set before you open any packs (and remember to craft the commons and uncommons before you draft).
This can be a tough rule to keep, especially if you’re on a bad run in your drafts. I’ve broken the rule more than a few times, and you probably will, too. Don’t beat yourself up about it! We’re only human.
One final note: Arena offers the option to buy a Mythic Pack. This is just a regular pack that’s guaranteed to have a Mythic instead of a rare. It’s 30% more expensive than a regular pack. These are not given out as rewards and can only be purchased. I don’t recommend buying them.