===== Answer =====

It'll take 3 rounds.

Number the coins from 1-7. 

Beginning: H H H H H H H \\
Round 1: flip 1, 2, 3, 4, 5 ( T T T T T H H ) \\
Round 2: flip 2, 3, 4, 5, 6 ( T H H H H T H ) \\
Round 3: flip 2, 3, 4, 5, 7 ( T T T T T T T ) \\