From personal experience I would have to say that you need to wait 72 hours. But remember you need to put tolerance into the equation. If you are taking 24mg of buprenorphine, even just 8mg, your tolerance will have increased since the last time you were using. I ran into this problem. I was on 8mg of Suboxone, hadn't taken an opiate in over a year and I decided I was going to relapse for a couple days. I waited 72 hours and took 60mg oxycodone. I thought that was a big enough dose since it had been more than a years since I last used but I was wrong, not only did I not feel anything, I was still feeling some of the withdrawals from the Suboxone (they kicked in about 48 hours after my last dose). It turns out 8mg of buprenorphine is stronger than 60mg of oxycodone. So be prepared that you are going to need a high dose to break through the 24mg of Sub that you were previously taking.
But 72 hours should be long enough to take another opiate.