We decided to replace the battery in an iPhone 6 and document exactly how it went. Quick backstory: Apple announced in early January that owners of the iPhone 6, iPhone 6s, iPhone 6s Plus and iPhone 7 could replace the batteries in their phones for just $29. Apple began replacing the batteries after backlash from customers who noticed that Apple had started to throttle the speeds of the phones, making them feel slower. There are rumors that it's taking up to months to get a replacement, however, so we decided to find out just how bad the situation is firsthand using both an iPhone 6 and 6s. Here's what went down.

The process

Locations with appointments to swap a battery.

First, we visited Apple's support website and selected assistance for the iPhone, chose the battery option and then selected "battery replacement." We decided it would be best to try to get the battery swapped in an Apple Store, and selected the "bring in for repair" option. Apple showed a list of locations where we could get the battery swapped as soon as the next day — including places such as Best Buy and mom and pop shops. The majority of locations didn't have appointments for a few days. We decided to try the 14th Street Apple Store in New York and made an appointment for the next day.

The experience

Walking into the 14th Street Apple Store Todd Haselton | CNBC

There's good news and bad news. Apple said it couldn't replace the battery in our iPhone 6 for about two weeks. An employee said that about 90 percent of his appointments are for battery replacements, which suggests Apple can't keep up with the demand right now. On the other hand, Apple said it could replace the battery in our iPhone 6s in just two hours. It was also eligible for a free battery replacement since it was purchased between October and November 2015. Phones sold at that time were more sensitive to temperature changes, causing them to turn off. As with most Apple repairs, your phone can't have excessive damage. The iPhone 6s we brought in had seen better days — and had a third-party screen replacement. It was also missing the screws near the charging port. The Apple Genius Bar employee was skeptical about the phone's third-party screen, and said there was a 50 percent chance that if they took the phone apart it wouldn't work again. If so, it would require a $129 screen repair, or we would have to replace the entire device for $299. We decided to take the risk — and it paid off. We picked it up later in the evening with a new battery.

Your mileage may vary

Waiting for our replacement. Todd Haselton | CNBC

Another colleague at CNBC said she took two iPhone 6 units to a Best Buy, also with an appointment made through Apple's website, and was told that there wasn't stock available to replace her battery. A call to Best Buy a week later confirmed there still weren't any batteries available. Your luck may vary depending on where you live and whether or not your nearby Apple Store has batteries in stock or not. If you live far away, consider calling ahead of time.

Our advice

This iPhone 6S went from 100 percent battery to 14 percent in four hours. CNBC/Michelle Castillo