Anywhere that won't cause attention. Don't spark up in front of the parliament, don't spark up in front of the police station, don't spark up in front of a school or kids park.
Most people don't care, they will just walk by. When you see someone smoking do you go up to them or continue on with your day?
Rarely will anyone go out of their way to point out a toker, or to call the cops on them. If they do call the cops, they have better things to do... they might come by and check, but by the time they do you'll be done and gone.
Be careful on small paths and stuff like that, cops will something walk or bike those routes. I don't know where you live, but here.... even those who are not legal patients... it's better to just look at the cop and say "Yeah I was smoking a joint". They're not stupid, saying you were not smoking when you wreak like weed is going to get you into more trouble. Always best to just keep your mouth shut if you don't have anything good to say.
^Obviously it depends on the context of the situation, always best to talk to your lawyer.
I'm a patient so I don't worry about when and where, but I do. Although I don't have to care, I respect others. There's this huge area with paths and water, a huge hill with a bench... so completely empty, and this guy decides he wants to come and play football with his two kids, exactly in front of where I'm sitting (probably an acre of space.). I simple stopped smoking my joint, got up, smiled .... walked away and continue to finish my joint.