Use Google Maps to check the route, driving time should be about 4,5 hrs if you choose the shortest way (+ some time in traffic jams).
I can't help with the car rental issue, it's always the problem with crossing borders. If you want to visit Dresden, you can leave your car there and go to Wroclaw by train (there are 3 direct trains daily, it takes 3hrs15mins).
Many car rental places will allow you to drop off the car in another country, but you will pay through the nose for it.
The route through Cottbus does take about 4.5hrs as rocking_pony says - the first 60km of road on the Polish side (i.e. heading in to Poland) is very worn and pitted so you can't go fast, but apart from that it's an easy drive.
Yup, ewainkrakow's right - dropping a car in Poland will cost more, that's why I suggested going through Dresden, leaving a car there and travelling to Wroclaw by train - the travel itself will cost less, you can visit this fantastic city but it will also take much more time.
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