For me, about the third day I felt a LOT better. That being said.. I've been wearing the ShrinkX which I swear makes an amazing difference; I bought mine secondhand on Kijiji for $30 and its so worth it. Though I don't know if my SPD is typical.. it was a combination of that and sciatica and my LO was resting on a nerve on one side and had his head turned up toward my hip, so I'm not sure if that factored in as well.
But basically the last month of pregnancy, I could barely walk some days. I was really surprised how much better it's been. I still get pains, but my mobility is WAY better. Though there have been a few times I push it and forget to get off the bed properly and end up waddling again, lol.