Although setting goals is as unique as our writing voice, I suggest setting several monthly goals. This ensures that you have another goal that creates forward movement in your writing career if you get "stuck" with a certain plot point or have to wait for research material to arrive from inter-library loan.
But don't be so strict with your monthly goals that if a writing opportunity arises with a short deadline that your turn it down. Last spring I interrupted my set goals to answer a manuscript call from an editor who needed short craft ideas. I did this because I've written for this publisher before, I enjoy writing short craft ideas and this call had a four week turn around time, so I couldn't put it off until the month.
I set anywhere from three to six monthly goals because this works best for me since I also work a full time job. If you have more hours to devote to your writing, you may want to increase the number of monthly goals you set.