How Long Does Trimethoprim Affect The Effectiveness Of Contraceptive Pills?

I've been prescribed trimethoprim due to continued UTI's, I was told to use extra precautions due to its effect on the contraceptive pill and that I needed to wait 3 weeks before it was back to normal. Over the past few weeks I have been taking trimethoprim, but it hasn't been daily, more like every few days, I'm wondering if i then still need to wait 3 weeks of taking it continuously before no extra precautions are needed?