It is best to wait about 6-8 weeks like you would with any other piercing :)
It varies from person to person, depending on the size of your nipple.
After 6weeks you can take it out.
You need to wait about 6 to 8 weeks after getting them pierced before you change them.
How long has it been pierced
You can get it done again once you're sure the piercing is healed inside as well as out.
As long as you are taking care of it nothing would happen and it should be fine.
Before you can change it out you should wait 3 to 5 days
6~8 weeks and even longer depending on how the piercing was done. As long as the mater is whitish yellow you are o.k. and that is normal discharge, it will ease up as the piercing heals and becomes seasoned.
Provided it was done properly and it is deep enough, it shouldn't reject and should last for as long as you want it.
Yes, as long as the gauge is correct for the tongue piercing - for example : nipple bar is 14 gauge - usual tongue piercing jewellery is 14 gauge and the length of the nipple bar is the correct length for the wearers tongue (this varies from person to person). Basically - if it fits - wear it!
When they're healed inside AND out; it'll heal outside first. Don't mess with them too much or with dirty hands because then you risk infection or migration.