Well first of all, the question is, do you get the full 6000kbps and if you exceed the threshold, will you still get full 3072kbps? So you should make speed tests on the stability of your line bandwidth first.
The next thing is, do you really need 1080p? The differences in terms of video quality between 1080p@3000kbps and 720p@3000 are pretty visible. 1080p doesn't garantee you a good quality at all! Downscaling to 720p at the same bitrate does look better in most cases. OFC the details are kinda sqashed a little, but that doesn't matter when it comes to your subjective feeling. Sometimes, increasing FPS to 50 or 60 will create an enhanced viewing experience, the video looks more vivid, but on the contrary side the encoder has less time to process each image, so sometimes the images look washy. So if you decide to have the best image quality possbile, use less FPS instead. For 1080p I would recommend the highest bitrate possible, which is 3500kbps for twitch (and which can be very CPU intensive, too). But in your case that wouldn't be possible, you would pass the threshold before you get to 1 hour.
On a weekday I would recommend trying to use 720p30fps@2000 - 2500kbps. That would still come very close to your threshold after 1 hour though. Another problem that you have to take into account is that with your stream you're sending other data than video (very likely for online games etc) as well which will cause traffic, too. So you need to make your own experiment on that.
If you have already passed the threshold, you should be able to stream at 2000-2500kbps (while reduced to ~3072kbps). I can not really say if 3072kbps is still enough for streaming and doing other certain things (I've got 50mbps up so I can't tell) but one problem that could occure as you are hard at the limit, is that frames could get lost if you're demanding too much from your line when you're already on the reduced state because the data just won't pass in time anymore.
One good thing: in the reduced state, there shouldn't be any latency issues for example in-game if you're streaming as long as you're not going too much over 3000 overall, as the reduction is 'artificial'. You are still connected with 6000kbps but 'artifically throttled'.. don't know if it's the correct word for that, I'm German sry :P But there's certainly a difference between a line throttled from 6k to 3k and a line that connects with 3k max to the ISP.
So in the reduced state your line should be more stable if you reach 3000kbps than a line with 3000kbps max upload.
I am not sure if you have the chance to pick another plan or ISP, but if you can, you should think about alternatives, if streaming is what you really need to do.
Concerning the weekends, I would say it is impossible to cover several hours with decent quality stream.
One thing I forget:
It also depends on which basis your ISP calculates your traffic. And on a weekday, what will happen if you reach 1200MB and in the 61st minute you are going to upload like 1000MB file. Will you pass the threshold by that or not?
