Before the project has really started — when you're still planning. This is when I can add insight into time requirements, different stages and setting realistic goals so we can measure the project's success.
But I also like to be involved in the execution phase. A huge part of any project's success depends on the finer details of execution, and this is where my attention for detail, organisation skills and my enthusiasm for getting sh*t done can really make a difference.
Finally, to mis-quote a German football saying: after the project is before the project. I see projects as cycles of work, so the testing, learning & evaluation afterwards are very important to me. I like to learn what went well and what can be improved — and fix anything I can optimise easily.