These are good questions.
First, how often depends on what your candle symbols show. And, if you can't read the symbols, then go with your feeling. If the protection spell weakens, you will just know it and will feel like either performing the spells again. OR get reinforcement by having a vigil candle set (you can do yourself or have MISC do it for you).
Things will just start not going as well (little things) and you will just know.
Here is a list of protection spells (and you can state the intent with your petition): www.luckymojo.com/protectionspells.html
If you are worried about life and death, you may also want to contact authorities if someone is threatening you and your family
Yes, you can perform the spell for a group of people.
And, definitely YES, blessing afterwards is perfect
Wishing you and your family the very best