Hi mercy, welcome to the forum!
First to answer your questions,
1. It should be days, weeks at most.
2. Get a 5 gallon fish tank, an aquarium filter, a heater, a thermometer, water conditioner, fish food. It will probably cost you up to $100 in total, but it is almost an one time investment. Afterward, you only need to restock water conditioner and fish food once in a long while.
3. Do not use bottled water. It might have different hardness and PH from your local water. Fish does not like large and quick changes in these readings. It can kill them. You need tap water used with aquarium water conditioner.
4. Unfortunately, a lot.
These pet stores are never good at giving the right advices when it comes to take good care of your pets.
A. You need a real
fish tank of proper size.
You need a minimal 5 gallon fish tank to keep stable water conditions. Preferable a 10 gallon. Without stable water temperature, PH, hardness, and other chemistries, the fish can be in shock easily and eventually die young.
B. You need a
heater.
Betta fish is tropical fish. Unless you live in an tropical area, you must have an aquarium heater for them. The ideal temperature for them is around 78~80F. The lower the water temperature, the lower their immune system will be, and the less activity you will see. You will also need a separated thermometer to monitor the actual water temperature.
C. A
filter system is a must for all aquariums.
It keeps up the
aquarium nitrogen cycle, or your fish will die to ammonia poison soon enough.
D. You need aquarium
water conditioner.
I prefer Prime.
E. You must learn the proper way for
aquarium maintenance.
Never change all of the water at once. It can shock and even kill the fish.
F.
Fish food and
feeding routine.
Betta has a natural life span of 5~7 years. Some reported they have lived up to 10 or more years. Anything short of 5 years should be considered "died young" as a result of less than ideal conditions.
I highly recommend you to return one of the two betta fish. And keep only one (only if) you are up to create an ideal environment for it. Please do not be discouraged. Almost all of us had to learn it the hard way at some point. We were all newbies once.
Good luck!
P.S. There are many less than professional advices out there. Please choose what you believe wisely.
