One of the metrics used to elevate your chances to receive leads is how many projects one completes. My conversion rate is pretty low due to the fact that I do not Submit projects unless I get paid or personally fix something. A great number of the leads we get are just fishing for a deal, sort of dipping their toe into a pool to test the water temperature. Many times I find myself (voluntarily or not) sharing guidance on how they can improve their website before I have a written commitment, after all, most of them are small businesses struggling to start. Is it realistic to submit a completed project when the only thing you did was to give advice (paid or donated)?
Great question!
I believe you can submit once you have completed the work for the client. So if I pickup a lead and its ongoing over several weeks and months and wait until its fully finished before I mark as complete.
Equally I have done the odd freebie domain connection they dont want any more help at that stage so I mark as complete. Sometimes further down the line they have reached out for help.
From my perspective its when I have completed the request initial request if that makes sense 
I do the same. If I help the client (paid or not) they have added me as a contributor to their site, then I submit the lead. Also for clients that have agreed to work with me, paid a deposit but aren’t ready to proceed (i slowly build the site out at their pace) I hide the site and publish to the free URL and submit appropriately as deposit paid. There are many out there that do fish around as you said, when you close as “client not responding” it does not affect your conversion rate. Leaving a lead open for a long time does. Hope that helps!