Years ago, when my Jabiru was new and I was young and silly, I flew for months with no tank vent at all! On a one hour flight, not enough fuel was used to notice.
Then on a nav flight with the CFI, we flew for nearly 3 hours and on coming home, there was a loud "bonk" as the top of the fuel tank was sucked in.
The cause was found to be a bit of packing polystyrene in the fuel vent pipe. Under the fuel fumes, this had become like chewing gum and it had blocked the vent. The engine never missed a beat though.