Bear in mind that if the helmet did its job most people would tell you that they did not even hit their head, or did not hit their head that hard. And the thin shells on most helmets now tend to hide any dents in the foam. But if you can see marks on the shell or measure any foam crush at all, replace the helmet.

The bottom line: Leaving your bike outside for a day or two won't do major damage. You may see signs of rust after a week of neglect. After one month in bad conditions, your beloved bike parts will start to degrade.Leaving your bike outside for a day or two won't do major damage. You may see signs of rust after a week of neglect. After one month in bad conditions, your beloved bike parts will start to degrade.

The most important rules to consider while riding a bike are:
Wear a Helmet: No matter what, always wear a helmet. Invest in a good quality helmet which fits you properly. Investing in a good helmet goes a long way.
Proper riding gear: Safety comes first, if you're going for long rides, always wear proper riding gear to avoid any untoward incident.
Don't be a squid: Ride safely and responsibly. Cutting lanes, overspeeding and reckless driving aren't cool, they're madness.