The life line of a hash tag really depends on engagement and how often people tweet using the respective label.
In most cases the length of an event, the exposure and promotion of the hashtag, the quality of the event and the follow-up from the event all come into play to determine the lifeline of a hashtag…
Typically event hashtags are at its peak when the an event is about to start, during the event and a few hours after the event. Typically this allow rooms for introductions, reporting of the events and sharing of information and finally room for networking and engagement.
Ultimately it is up to the event organizer to keep the attendees engaged and deliver compelling content and resources for attendees to go back to the hashtag.
Hashtags can be a great way to follow and network with anyone a part of an event and build a list after engaging with the audience. Twitter of late seem to show hash tags 7-8 days out and any older are typically archived.
Despite such benefits there are very few event promoters that actually take advantage of such an underrated tool. If someone is using your hashtag and engaging in conversation this person has almost put a sign that says that they could be a potential customer for you in the future. This is unless you put a bad event and this can then turn into negative reviews and comments from an unhappy audience. Either ways both positive and negative reviews are shared on hashtags.