I was a long time 3ds Max user who stopped doing 3d for quite some time. I am recently getting back into the scene and has seen much changes. I've yet to decide on using a software, but Blender is one of my top choices currently. One thing that does concern me is the open nature of Blender. Although it can be good for development, I am concerned that I have to be constantly up to date on its features. For example, if I pick up 3ds Max today, most of its main functions stay the same. I am not certain about Blender. So my question is, if I decide to learn and master Blender, but may leave it for a while only to come back later, will its main interfaces still stay the same or not? I understand this is impossible to predict, but just give an opinion backed up with good reasoning. Thank you.
Other than a very big change from 2.49 to 2.5, the interface and main functions of Blender have stayed the same for a long time.
The logic and workflow in which Blender operates seems to be very stable, and works perfectly across Operating Systems. It's worth noting tha it runs smoothly on a very wide variety of hardware. Modest or old computers will work just fine but it will make good use of the resources of a powerful computer.
As most programs, blender keeps improving with some eventual breakups and minor changes; but backwards compatibility in blender is superior to most software out there. It does not abandon its users to their own devices or makes the files incompatible to in order force users to use the newest version. Blender, being free/libre and open source, does not have to respond to a particular market share or stock holders... There is no planned obsolescence for monetary gain.
Big changes are made mostly to accommodate newer technologies or to make the program more versatile (and sometimes easier to use).
Blender does not force its users to monthly payments, nor does it have a "light" (crippled) version, an "intermediate", "pro" or creative suites for VIPs. The same program can be used for a wide variety of purposes, from films and games, to 3D printing or Architectural rendering, or math or science visualizations.
So many people are using blender for all kinds of uses now, that commercial rendering engines are making ports to render blender projects. I can only see that kind of support growing, as blender is indeed getting wider recognition as a serious 3D tool. There are also some companies creating modified versions of blender for web content creation, biology scientific modelling and easy game asset creation. There is a growing a number of render farm services and cloud computing companies that support Blender as well.
Something worth mentioning is that the Blender Foundation has kept every version of the software (dating to v1.0) available for download. So even if your file were to be outdated, you can always go to blender.org, download the version used to create the file and retrieve your information.
The path for Blender's future seems quite bright and the developers are very open about their intentions and their road map.
But foreseeing the future is beyond the scope of this site.
Will blender's technology outlast that of the blender in your kitchen? Probably not.
In the meantime if someone out there finds an add-on or a modifier to render the future in real-time please post it here.
In addition to cegatron's answer, it's worth pointing out that just because Blender is open does not mean it lacks direction. The developers meet weekly to discuss progress, and the team sets long-term goals. The big difference is that you can be a part of that process, if you want to be.
I'd like to also point out that there is a potential that the left mouse button will replace right click. This might be a bit of a change. Otherwise I don't think to many things could change within Blender (obviously apart from features being added). Also as you said that you previously were using 3DsMax, there is an option on the Splash screen to set the Shortcuts to be like 3DsMax.