A month to a year
The human mind is incredibly flexible. Via association we can learn a great deal. When you start riding a bike or car you can see this very well. Moving your arms and legs in certain ways creates desired movement. We haven't evolved with these movements, yet we can learn them. Even better is controlling a computer for example. Moving your fingers on a keyboard or a mouse makes the computer do stuff. You can look around in a computer game, or type and select whatever you want in a word document. Finally, you can learn to fly an FPV drone. You put glasses on and get feedback of the camera. Moving your hands you can fly any direction you want. At the start it might be difficult, but with extensive training you'll be able to control it very well in a month, or for slow learners about six. These show that you can learn new abilities and movement by moving your body in a certain way in specific situations.
Even more impressive is doing something that flips it all upside down and left side right, going against what you've learned. A man made special glasses that flip and mirror the light coming in. After three months he was able to drive a car without problems. Interestingly he had trouble adjusting when he removed the glasses. The more he started switching, the easier it got.
For your questions Starfish Prime already has a great example. These monkeys receive a neural implant, either EEG or some evil spiked things into the brain. These can measure the electrical impulses of multiple neuron. They first try to match some signals in the brain, like moving up with your arm should move the arm up. Then the rest is just the monkey learning. A problem is noise of the signals, but we're getting better at it. But this noise is visible in the gif of the monkey moving the arm. It isn't fast and controlled yet. We're getting better at it though.
Interestingly EEG headsets exist that give you control over a mouse on your computer. Again, the movement of the mouse is mapped to a certain function of your cortex, like moving your arm. At the start people often need to move the arm in conjunction with the mouse. Interestingly at a certain moment these people start to think differently, and are able to move the mouse without moving the arm. That despite it being the same brain patterns on the cortical level. This shows that we're able to learn that some brain signals for an arm can be used for something completely different than moving an arm.
The conclusion is simple. Via association it will be relatively easy to learn a new body plan, like longer arms. That is very simple compared to flipping your world upside down and mirroring it, or moving a mouse along a screen. Even better. Most people already experienced getting longer arms during puberty! Although it might be a temporary and gradual thing, it does happen and is learned.
Now the kicker. EEG and many of the current brain pattern recognition is still in the starting phases of development. They are crude, relatively inaccurate and prone to noise. A suit is more like a keyboard and mouse, so you're able to control it without noise. If you then see through the eyes of the mech, you'll certainly be able to learn this new body plan quite quickly. Especially if there's bio feedback (you feel what the controlled item is feeling). With the mech and an intensive week you can probably get ok in controlling it. In an intensive month quite proficient and in two you'll think of it as a second skin.