Depends on the amperage you are applying to it. Read the charger manual.
That depends on the amp-hour capacity of the battery.
Depends on the amperage of the charger. A 10 amp charger will charge it in about 1 hour.
can a 12v lawn mower sealed battery be charged with 12v battery charger use on cars
If it is totally dead charge it for 2 hours at 10 amps.
No, you can not charge a 3V battery with 12V charger.
No
Till the battery is fully charged. -Could be 4-12 hours.
You can safely charge it with a 15 amp charger. You can also charge it with a 1 amp charger it will just take longer. Do not overcharge the battery.
A good auto battery will hold it's charge for at least 6 months or longer.
No.
No
No, you need at least 12v to charge 12v. Most alternators charge at 13-14 volts.