If you fly on these fares: which are also considered discounted:
K, H, V, N, U, E
Then you will get 200 points for the the roundtrip flight. You will need 1400 points for a free bomus ticket. So you will need 7 flights.
If you buy in thiese fares it will take you 14 flights since you get 100 points for the roundtrip: T, L, G, O

Best option is to fly in W,Y, S, M, Q, B. There you will get 400 for the roundtrip and you will probably achieve silver status after a year.
Chances are if you only come in the holidays you will not have the deeply discounted fares available anyway.
Ask your travel agent what fare she booked you in the past. N? H? B? or M?