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Hi Glen.
The question of "when is the primary mirror dirty enough" is a fickle one.
Shining a torch down the tube will show up every speck of dust, but the amount spread over the entire surface wouldn't cover the head of a pin. It looks bad, but that's all. Only when viewing through the eyepiece, you notice a degraded image, (stars not sharp dots, lack of structure in star clusters, the Moon lacking sharpness when looking along the shadows at low magnification, etc. And you know that collimation and seeing conditions are good.)
If the images are still good, then think about cleaning the primary mirror in 12 months time, unless something is spilt on it. There is a good article on mirror cleaning in the "projects and articles" section. I hope this helps.
Cheers!
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