In short, no you do not need to provide a "key" but you must provide some way for them to come and go without being incumbered. We've stayed at several Airbnbs where the hosts just leave the front door open so guests can just come and go. Some hosts install keyless entry systems like keypads that allow the guest to open the door. You can choose to stay at home the entire time a guest is there to personally open the door for them, but it sounds like you work outside of the house, so this is probably not possible. Most hosts just hand over a key and trust the guest will treat their home with respect and be responsible to return it. In the hundreds of thousands of bookings each week on Airbnb, this works just fine
We provide a key to our home and have an alarm system. We change the alarm code to match the guest each time so, even if they make a copy of the key, they will not be able to return to the home without setting off the alarm.
As for "when" you pass them a key, you need to set that up in the check in time. We have right in our listing that we must be home to provide a them a key. If you need to be back from work, set the check in time to start when you are home - 5:30 or 6pm. However, if you use a lockbox, you can send the guest the lockbox code a few hours before check in so they can unlock the box, retrieve the key, and let themselves in. Same with a keypad. Some folks arrange with another host, family member or neighbor to be the "pick up" for keys but, personally, I've always thought going someplace other than the host's home to pick up the key is uncomfortable.
When you provide a key, you will want a system for returning the key too. If your guests are allowed to check out after you leave for work, you'll want some way for them to self-service a return. Some hosts ask that they leave the key on the counter and lock the door behind them. You have a key of your own so you can come home later after work to retrieve it. Using a lock box, the guest can return it there. We try to arrange to be here for check out to inspect the room and ensure they have not left anything behind - and retrieve the key. But when a guest must leave very early in the morning, we don't get up, they lock the door behind them and drop the key in the mailbox. Our mailbox has a lock on it, so it's safe until we get it. Even with these measures in place, our keys have occassionally gone home with our guests. They then have to send them back to us. But if they don't, the alarm code being taken out of the system will keep them from coming back or forcing us to change the locks.
Hosts who provide keys do, occassionally, have to change the locks if the key is lost during a guest stay. If you have a security deposit and declare that you will charge for a lost key, you can request to have funds from the guest for doing this. But this is really a pain, not only to have the locks changed, but getting the security deposit to cover it. You can read more about this in the Help Center - there is an entire proceedure to recover for losses.
In summary, you have a lot of choices on how, but you must provide a way for your guest to come and go from the property with ease.
As for limiting when the guest can come and go, you can try this but you must tell them in your listing that they need to be out each morning by XX and cannot come back until XX. You will severely limit your bookings and guests who don't read all of the details will not comply automatically. Are you prepared to wake up people every morning to get them out of the house before you go to work? It's really not reasonable to do this. If you can't fathom people being in your house unattended by you, you probably should not host and invite strangers to stay with you. Airbnb is based on trust and mutual respect for both the host and their home. If you cannot trust the guests, then Airbnb isn't for you.
Good luck.
