Outlook doesn’t support irregular working days and hours. So if your regular working hours are for instance from 9AM till 6PM but always have the Wednesday afternoons off starting at 1PM or starting an hour later on Thursdays and Fridays, there is no way to set this exception in Outlook.
A workaround would be to simply add a recurring appointment and name it something like “My contractual free afternoon” or “I’m starting an hour later today” and set its “Show As” option as “Out of Office”. This can prevent someone trying to schedule a meeting with you on that time by mistake.

If you have a fully irregular working schedule, where for instance your working hours and days are different each day and/or week, there is not really a best practices. One method that might work for you is setting Outlook’s default working days and times to your companies main working hours and block off all the hours that you do not work with “Out of Office” appointments. This might be time consuming and quite cumbersome but so can changing meeting invitations; it really all depends on what works best for you.