Working the Lagom way means taking as long as necessary to get the task or project finished. That doesn't imply working too little; after all, you need to get it done, nor working overtime regularly. Adjusting expectations and setting realistic goals is essential to developing the desired (and possible!) work-life balance. Ask yourself, "Instead of working long, how can I work better?".
Moreover, bear in mind that the best you can do is usually good enough. We should learn from the Swedes how to get rid of the unattainable goal of perfection.
Here's an interesting video that teaches us how to balance life through Lagom; it focuses on Lagom at work between minutes 4:05 to 7:50. It's worth watching!