For Dragons and Barons, any time you have a numbers advantage or can get it without the enemy noticing. Without them noticing is the key here so as a jungler you should always carry a control ward with you to make sure the enemy isn't watching you take it. If you have a control ward and can solo dragon, you can really do it whenever you want to as long as you know you haven't been seen. It is helpful if you see the enemy jungler towards top side and bottom lane is busy doing their farming thing with everyone in lane. Never risk it when your bottom lane isn't there but their's is.
For Baron things are a little trickier because there aren't many champions who can solo Baron with their normal builds. To get Baron there is always something you should ensure; the enemy jungler is either dead or seen on the map somewhere far away from baron. If your team consistently wins team fights then you can force a baron when everyone is alive. Generally this isn't a good idea and should only be done when most or all of the enemy team is dead. A great time to get Baron is if you ace the enemy and push down the middle inhibitor. Immediately following the inhibitor is usually a guaranteed Baron if your team is healthy enough to do it.
Team fighting and objectives go hand-in-hand. Always ask why you are fighting. Of coarse this is only the case in a close game where fights can go either way. If you are ahead and pretty much guaranteed to win then force fights at every possible chance. In the mid game when respawn timers are still under 30 seconds, it is important to make sure you can actually do something with a won team fight. If you don't have minions pushed close to towers and Baron and Dragon aren't up then there is absolutely no reason to fight. Split up to push the minion waves and then regroup so if you win a fight you can actually do something with it. If you are on the defensive, you should never fight outside of tower range. I see it too often in low elo that people want to chase someone into their jungle when they are defending. Even if you kill that person, you gain nothing because there are no towers you can take and Baron and Dragon are too much of a risk.