It depends on the skill you spend points on, for example 'Youthful Lungs' maxed out 5/5 gives you 25% max stamina, so each point you spend gives you 5% increase.
'Crawl before you walk' maxed out at 5/5 gives you 50% increase in downed movement speed, again each point spent is 10% increase.
All the skills have different percentages. There's a few YouTubers that have painstakingly gone into the practice area to test each of them out and used a timer to see difference in some of the skills. It's worth checking out if you want to get the best out of the skill tree, depending on how you want to play the game of course.