Nobody's going to be able to give you any sort of decent rendering time estimates since everything from project to project is going to be very different. The only way somebody would be able to help is if they had the exact same system you have and ran a benchmark AE render on it. But it's difficult to come up with a good benchmark since there are so many different things that render so differently. Blurs, HD footage, Particular...all can render faster or slower depending on numerous things and besides, even on computers with the exact same specs, things like which other programs (such as antivirus software) you have running make a difference.
That being said, at least you will be able to run AE on there.
It's better than my old home system which was running a Pentium 4 (no hyperthreading) and 1 GB of RAM. :P
I created full-frame 1920x1080 HD on my old system in AE 7. It was quite a bit slower than I liked, but at least it worked. I have since upgraded to 6 cores and 24GB RAM at home. :)
I would strongly recommend against getting a laptop for AE work. You get more processing power for your money in a desktop, plus desktops are easier to upgrade. For example, it would be better to run your operating system and programs on a separate hard drive from your video assets and renders and you can't do that in your laptop.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.