What is a reasonable salary merit increment to achieve per year?

It's honestly not within your control. I think its something you gotta experience when you are in the company / seek to understand from other employees. Individual companies have different increment structures and considerations.

​

Go in with an increment expectation based on what you deliver to the company - that's how you negotiate rather than saying "The industry average increment is 5%, why did I only get 3%?" Increments needs to be justified to one's performance, deliverables and contributions.

2-3% mostly, and can be lower depends on economy situation as well.