How many cycles on Letrozole

Ronda • TTC #1 since Sept 2013.
Hi ladies,
My husband and I have been TTC for over two years with three MCs. This past month was our first on Letrozole. My Dr put me on that first due to my age (35). While I ovulated and my progesterone was good I'm BFN at 13DPO. Pretty sure I'm out as I feel AF trying to start. My question is how many cycles of Letrozole only should I try before asking to get more aggressive with fertility treatments?