As many people have already said, it's best to wait as long as possible before inducting oneself. However, the amount of time can very due to the half lives of different opiates. For example, methadone has a 36 hour half life and it's recommended that patients taper down to a dose below 30mg and then wait 3 days before getting on buprenorphine, whereas with hydromorphone or fentanyl, someone could probably induct oneself after 12-16 hours after the last use. It sounds like you're using heroin though, and IME I always tried to wait
at least 12 hours, though the longer the better, preferably 24-36. Basically, the longer you wait, the more effective the suboxone is going to be, and less likely are chances of precipitated withdrawal. If you dose the suboxone at 7 a.m tomorrow morning you should be totally good to go.'
Also, although precipitated withdrawals are really terrible, they only last a couple hours at the most (assuming you're not transitioning from methadone). If you can get ahold of some Gabapentin like Phil stated, along with some clonodine/catapress, you should be able to fend off the worst of the withdrawal symptoms, clonodine is especially effective in terms of a non narcotic withdrawal medication, it's very under-rated.
I'm not sure what your habit is like, but assuming you're able to wait 24 hours, I would start with 2mg, and give that a good hour to kick in, after which you can asses how you feel, if you need more, or if that dose was too high etc. I'm not sure if you're just planning to do a quick detox, or whether you're getting on subs for maintenance, but either way, the lower the dose the better IMO, though sometimes the first few days people need higher dosages like 8mg before they stabilize, and for some I suppose the fact that at high dosages Bupe can block full agonists can help with temptation as well.
Also
Check out this thread, for information on how to more efficiently absorb buprenorphine via sublingual administration by creating an alcoholic solution. The thread provides a lot of good information on how to boost the bioavailability of sublingual buprenorphine by creating an alcoholic solution. If affording the subs is a difficulty, or you want to stretch the subs out more, or just get a more consistent 'effect' through sublingual suboxone, it's really useful information.