I was just looking through clans in the saloon window but i mistakenly sent a request to one of the clans. So if they accept can i leave the clan right after i go online again? And also does the requests dismiss automatically if they are not being accepted nor rejected?
If you got accepted in that clan, you have to wait 72 hours from the moment you have joined before being able to join another clan. Of course you will always held the option to leave, just would be forced to stay without clan until the duration is over.
The request is dismissed only in case there are no officers online to receive it. If there was an office online around that time, he will be able to accept you or reject you at any time in the following days, while other officers wouldn't even be able to view that request.