Andrew,

The sink rate for tips is expressed in Inches Per Second (IPS).

If you want your line to get to 7 feet deep, and it's a 6 IPS line, that's at least 14 seconds. If there is current, you'll want to think about that, how far the line/fly will move during that time before attaining the depth you want (you have to keep the current from dragging on the line or it will cease to sink).

In any event, you'll be happiest with the fastest sinking tip your rod can throw. You can control the depth by when you begin the retrieve in still waters or tighten the line in current.

Good Luck!

Buddy