These are my GENERAL rules for skill checks in combat, not all checks or all situations will always follow these rules.
STRENGTH
A Strength Check will generally be an action. Athletics is the only strength skill, and I see using your brute strength to open a door, pull a level, or similar to take some time. Remember a round is only 6 seconds long.
DEXTERITY
Dexterity Checks consist of Acrobatics, Slight of Hand, and Stealth skill checks. These will be a bit more variable. If you are proficient in Stealth or Slight of Hand, those checks will generally be free for you. If you are not proficient they can still be free, but with Disadvantage as you aren't skilled in those and you aren't spending time to get the best result. You may choose to use your action for those skills and roll normally if you'd like. Simulating you taking your time on those skills.
For example a Rogue with proficiency in Stealth would be able to stealthy move up to a target and attack them. But a fighter without that proficiency in Stealth wouldn't have the same know-how to quickly and quietly get up to an enemy to attack. He can still make the check, but is at bigger risk at making a noise or alerting the enemy.
Lastly for Dexterity Checks are Acrobatics checks. These will be free, but a bad roll has a chance to put you in a bad position such as potentially falling prone.
CONSTITUTION
There are no Constitution Skill checks.
INTELLIGENCE
Intelligence Skill Checks are free, as they are recalling information.
WISDOM
Wisdom Skill Checks (Animal Handling, Insight, Medicine, Perception, Survival) will be similar to Dexterity, they will be free with proficiency, and free WITH Disadvantage if not proficient, with the option to spend your action to roll normally. The exception would be Insight, that will be a free check.
CHARISMA
Charisma Skill checks will generally be free (Deception, persuasion, intimidate specifically) as speaking is a free action. Though a Performance check will normally cost an action.
