Any time is a good time for affirmations. I would have to say though, that the time your mind is most likely to allow the positivity to take root in your consciousness is when you are calm.
Some styles of yoga incorporate pranayama in the beginning of their practice; the Yapana® style is to do physical practice first, then a brief still time before meditation / breathing. That way the mind is considerably less agitated. It’s like weeding a garden before you plant. The new seeds shouldn’t compete for nourishment with the weeds.
Try a 5 minute mindfulness of breathing then repeat the affirmation “Everything comes to me at the perfect time” and let it work into your psyche – this is a good recipe for your mind-body-spirit