I don't know about a "month" but the issue is that Ethanol is harmful to the rubbers found in carburetors and fuel systems. Have you ever wondered why not all vehicles are capable of running E85? It's not just the computer that has to be capable of adjusting for the increased octane, but he fuel system components in E85 are completely different (materials) to handle the added ethanol.
So, while virtually all engines intended for regular unleaded, containing some small amount of ethanol, should be fine, it does not mean that the machine WONT have some small adverse effect over time. This is why most engine manuals for small equipment specifically direct consumers to use a fuel additive that combats the harmful effects of ethanol.