That doesn't really tell me much.
I mean how many units it is depends on how much water you're using, and how dark it might be depends on the heroin.
A lot of our heroin where I live mixes up totally clear. 30 units could have a half a point or it could have half a gram.
Anyway, how long it'll likely last also depends on your tolerance.
For me, if I took as much as I would normally take, it'd last about 8-12 hours before I started getting noticeably sick. But if it were say, half that amount, that time could be cut in half.
If I may ask, are you trying to get off heroin? Or just get through the time till your next shot?