Hey Anna, This is one topic I actually know something about

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If you want to see a significant improvement, once a week on the grind is fine providing that you are also doing another activity (cyclists seem to do well). Even if you choose to do nothing else you'll probably find yourself knocking off 20 minutes over the course of the summer if you stick with it on a weekly basis. Twice a week is better but start slow (once a week for a few weeks), this type of schedule could probably take you under an hour by Sept.
It actually took me three years at twice a week (in season) + gym in off season to break 45 min. That being said my ex wife broke 45 mins in three years at twice a week with no additional offseason stuff...I think spite and vengence was involved here but that's just a subjective observation. Getting below 45 becomes very very difficult imho. I've hit 39 and change a few times but will still float back to 45 if its hot or I feel off.
I wish you the best of luck and remember that even if you find it getting tedious, its a whole lot better than a stairmaster in a gym.