You can picket anywhere you like until someone complains, and you can keep on doing it until the cops show up. If the cops tell you to stop, you should comply- unless getting arrested is part of your plan, as happens sometimes. Remember that the police can arrest you at any time for any reason or no reason- it’s easy for the DA to come up with something to charge you with, and if they let you go they usually don’t even have to worry about charging you with a crime.
If you seriously intend to do this I strongly recommend you don’t go alone. One guy picketing is just a sad, pathetic kook. Before you start, make contact with an organization like the ACLU or another such organization sympathetic to your views which has legal representation. Find one with a branch that is local to you, or rather local to where you intend to have your protest. They or their lawyers can explain the legalities to you much better than Fluther or any other internet community can- and they might even defend you if you are arrested.