Brisket cooking time 6lb 300-350F

at 300 to 350 probably around 4 hours. foil @ 2 hours and start checking after 2 more for tenderness.

I would start checking it at 3 hours.

Start it a little early and let it rest. Use the probe goes through like butter test instead about worrying about time or temp too much.

You can foil it or not... sounds like you might hava a flat, That's the lean part of the brisket. At that temp you might want to foil it at around two hours or whenever you have developed some nice char on both sides. Leave unwrapped the whole time at 300-350°, and you might find yourself with only a thin layer of juicy meat between two heavily charred sides.

For myself, I don't foil it, but I cook at 225-275. I only buy packers, and even if I separate the point and flat before cooking, I stack them until the last several hours so the flat doesn't end up with too heavy of a char. The point's a whole different story. I'll leave it in or put it back in to develop more char for a couple hours after the flat is done. Those fatty points are really good with nice juicy(fatty) meat between two nice crunchy layers of char. Makes for great pulled beef too!