Two very common questions I get when it comes to memory...
- On a new Mac, how much memory should I buy with it (or is just the base amount good enough)?
- On an older Mac, when is it time to upgrade my memory and add more?
The short and easy answer to the first question is "As much as you can afford". If possible, max it out. If 16 GB is the max option given to you when purchasing your new Mac, then do it. This will, among other things, definitely future-proof your computer. Chances are, you will have and use this computer for several years, and you probably don't know for a surety what you will be using it for 3 years from now.
The one glaring caveat to that mode of thinking: if you can't afford to max out your memory, then scale back to something a little more reasonable. The other caveat is if you only plan on using your new Mac for very basic use (email, Internet, word processing, etc), then it's obviously overkill to put 16 GB of memory in the system. You'll never take advantage of it. In that case, the base 4 GB or 8 GB is most likely plenty. One last thing to consider is that many of the new Mac portables come with a fixed amount of memory, and it's either difficult (as is the case with the MacBook Air) or impossible (as is the case with the MacBook Pro with Retina) to upgrade your memory after the fact. This may play into your decision at order time.
As for question #2, we need to look at how you are using your memory now. The best place to do this would be the Activity Monitor (Applications > Utilities > Activity Monitor). I've reviewed this utility in some detail in several previous posts (do a search for "activity monitor" for more detail), but for the sake of answering this specific question, I want to hone in on the System Memory tab.
Here, you will find several pieces of information that will be useful in determining when it's time to upgrade your RAM.
First, let's look at overall memory usage. The image above is a snapshot of my MacBook Pro with Retina. It has been running for about 3 days non-stop now, and you can see my memory allocation breakdown. As you can see, I still have a fair amount of memory that can still be allocated for additional tasks (in other words, I haven't used all my memory yet). Considering that my system has been on for a few days now without being restarted, it's safe to assume that this is typical for my use pattern. If this graph showed no green, it may be time to increase your RAM.
Looking at overall memory usage isn't necessarily enough to help you make this decision. We also want to look at paging. In the snapshot above, I have 1.33 GB of page-ins and 0 bytes of page-outs. Page INs gauges the amount of data written to memory from long-term storage (Hard Drive or SSD). This can be loading an application, opening files, etc. As you can imagine, page OUTs gauges the amount of data being written to long-term storage from memory. Typically, paging OUT occurs when there isn't enough memory to store your system/application data in memory, so it temporarily stores it on your Hard Drive or SSD (using it as temporary memory). On my system (which has been running for 3 days now), I have 0 page OUTs, indicating that I have a healthy amount of RAM. If the amount of page OUTs came close to, matched or exceeded the amount of Page INs, then it's probably time to consider upgrading your RAM.