Let's suppose I start such a list, and pass it on to 5 friends, and ask that they continue the chain. By the time there are 15 names on each list, up to 36,000,000,000 emails will have been sent, and each one one of them has my name on it. Perhaps instead of trying to count how many people have signed the petition, it is intended to forward the returned lists on to the person or organisation being petitioned. Supposing everyone passes the list on to just 3 other people, and that all the lists are returned, each with just 15 names on them. Forwarding on all the returned lists at the rate of 1 per second would take about 2 months.
Email petitions tend to continue circulating indefinitely.
Some of the ones I've been sent recently I first received more than a year earlier. And usually the older they are, the longer they are, and therefore the more time and space they take up. The address of the petition page can be passed on using relevant mailing lists that people have subscribed to, by links on sympathetic web sites, or via messages to the appropriate newsgroups. There is a world of difference between this and using a direct email technique which is unlikely to accomplish the original purpose of the petition. Ironically, it is often those who are most concerned about pollution, environmental preservation and human rights in the physical world who are responsible for this type of littering in the virtual world.