for example : I added proposal ("Tee and Coffee :D"), nobody follow it (except for me), nobody closed it. My proposal has small activity. When (if at all) my proposal will be deleted ? Who will delete it (person or algoritm).
There are three processes in place to clean up proposals that just don't seem to be going anywhere.
Closing a Proposal After 1+ Year in Definition or Commitment — On occasion, we have to uproot and close a proposal which has been painfully slow at gaining support. Typically, there is nothing inherently wrong with the subject… but after 1+ year in Definition or Commitment, you have to determine if the proposal still has the support it once had. Proposals that take longer than one year in Definition or Commitment are subject to closure.
Abandoned Proposals — As part of the routine maintenance of Area 51, proposals that have ABSOLUTELY NO ACTIVITY for a period of 30 days can be closed as not a viable proposal. This is an extremely conservative measure. It only happens when a proposal receive absolutely NO activity at all — no new questions, no comments, no voting… absolutely nothing. It takes only the tiniest bit of activity to keep the proposal alive.
Once a proposal is closed, it will eventually be removed by the routine maintenance processes of Area 51. This typically occurs after about 30 days.
Non-Starters — New proposals are often submitted and quickly abandoned for one reason or another. Proposals that don't see even a minimal level of activity are subject to removal.
Whatever the case, it is perfectly acceptable — and sometimes even recommended — to start a closed/delete proposal again if the situation calls for it. Circumstances change and sometimes old ideas find a source of renewed interested. I would also note that closing and restarting 1+ year old proposals has spurred some of our strongest launches on the network to date!