Is there an ideal time to use marketing?

Solution 1:

According to the wikia success guide:

When you self-publish medium and large games, market like this: 500K Campaign at the beginning of Stage 2, then 50K Campaign every now and then (find out for yourself the interval, if you do it to soon you will see no effect, if you do it correctly you will see immediate increase in hype by several dozen). If you want more hype you can do another 500K Campaign at the beginning of Stage 3, but I don't think it's worth it.

I personally found that a large campaign at the beginning of development usually suffices and additional campaigns during development add little value.