Hmm - depends how thick it is - but even then it's difficult to say unless you have an oven thermometer, oven thermostats are often about 20C out!
Just found a hugh fearnley-whittingstall recipe:
2kg piece boned, rolled beef brisket (fresh, not salted)
4-5 garlic cloves, bruised
Good handful of thyme sprigs
Olive oil
Salt and freshly ground pepper
About 400g baby onions or shallots, outer skins removed
Preheat the oven to 200C/400F/gas mark 6. Put the brisket in a large roasting dish. Tuck the garlic and thyme inside and under it. Pour over two to three tablespoons of olive oil and massage into the meat, then season well. Put the meat in the oven for 20-30 minutes, then remove it. Turn down the oven to 130C/250F/gas mark 1/2 , cover the meat with foil and return to the oven for four hours.
I'd follow that if you can - should give a nice browning to the outside with the initial cooking and then slowly cook the rest