I don't think there's a normal amount of time. Some keep it active for years, some for months, some for a couple of issues, some don't even finish with one issue. SPINN can be managed by literally anyone from the region, so it all depends on who claims it and how dedicated they are.

On a personal note, I enjoyed the SPINN versions of late 2012-early 2013 and late 2013-early 2014. Those two were generally active and published interesting pieces, plus they had an active and inclusive staff. But nothing will ever surpass the Southern Journal interviewing Max Barry.