Did you know that Apple's sales of iPads are declining? It's a curious phenomenon that industry pundits and analysts didn't understand until they started talking to users. What they found out surprised everyone and eventually put a dent in Apple.

When asked, users said that their tablet was adequate for their needs. That sounds innocent at first. After all, most folks get a tablet for a few key functions - Web access, email, reading, and perhaps casual gaming. In these first few years for tablets, not many were buying them for use as fully-capable PCs. Most tablet buyers already had a PC and used it for heaving lifting.

Because smartphones have an expected life of about two years, Apple can be forgiven for thinking that the cycle would be the same for tablets. Didn't happen. The users said "this is enough, it does what I want" and just keep using the things. My son and his wife say exactly the same thing about their older iPad and are not swayed by any of the premium features of subsequent models.

What does this mean for you?

Quite simply, it means if you buy an iPad or Android tablet you will probably keep it for longer than you think, certainly longer than your current smartphone. And that means you should think long and hard about buying it for that longer period.

For most tablets, options are simple and usually limited to one - storage. As a quick example, the list price of the iPad Air 2 is $500 with 16GB of storage, $600 with 64GB, and $700 with 128GB. You might look at the least expensive model and think that 16GB is plenty for now, something that might be true if you planned to buy a new iPad two years hence. But if you want to maximize the life of this purchase, you want to make sure that you have enough storage to handle your needs for three, four, or maybe even five years.

You would be right if you thought that $100 is a lot to pay for that next 48GB of storage. Here's the catch - what does it cost to buy another iPad two years out? Apple has an extraordinary track record when it comes to keeping prices up. Taking liberties, let's assume your next iPad will cost $400 with 64GB. Now we can do some math:

• Case 1: Two iPads each kept for 2 years, $900 total for the four years or $225 per year.
• Case 2: One iPad kept for 4 years, $600 total or $150 per year.

Unlike traditional laptops or desktop PCs, tablets can't be upgraded with more storage (or main memory). When you've exhausted storage on the device, your only option is another device with more.

The bottom line is that you must think ahead when buying a tablet and that thinking needs to be about storage. Today, I would not consider buying a tablet without at least 64GB built in and I would not consider buying a phone without at least 32GB. Every two years, up those numbers by at least 50%.

My advice here is not brand-specific. However, I have some thoughts.

I can't understand why Apple has not put some kind of card slot into iPads, especially the iPad Pro. I have a two year old Dell Venue 8 Pro (cost: $230) that came with 32GB plus a micro-SD slot, into which I instantly added a 64GB card. Today the $500 Microsoft Surface 3 has 64GB and a slot. The $600 model has 128GB.

• Case 3: One Surface 3 kept for 4 years, $600 or $150 per year but with double the storage on day one.

Remember: it's all about storage.