I'm not sure about DLR, but at WDW, the Love Letters and Duets tend to stick around for a few days/weeks. I've seen the Q of Hearts Duet stick around and overlap the next release! So I doubt they simply fly off the shelves at DLR. The LL this month is Donald/Daisy, so I doubt it will fly either. Being at WDW, I have no concept of the Decades pin. I will add, though, you're only allowed to buy 2 per pin/per day/(per store). So if you're only visiting one shop with LE pins and you're alone, you'll only be able to get two of each. Just something to keep in mind.
In general though, I haven't seen many pins sell out in their first days at the parks (except for things like the Piece of Disney History pins, those are usually pretty quick). So I think you should have some luck on Friday night. 🙂
~Merlin