When you are climbing certain terrains that require an Athletics check, do you have to roll the Athletics check on every turn you climb? Or just once?
The rules don't specify that
• You attempt to climb a sheer or slippery cliff, avoid hazards while scaling a wall, or cling to a surface while something is trying to knock you off. (Basic Rules p. 62)
The PC rolls an ability check when the DM calls for one.
An ability check tests a character’s or monster’s innate talent and training in an effort to overcome a challenge. The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure.
When the outcome is uncertain, the dice determine the results. Basic Rules p. 61
Ability checks, if you are the DM: roll once (recommendation)
Based on how our games have gone over the past 5 years, I recommend to you the following if you are a DM:
For a climb, check once, and adjudicate the spot as "half way up/down" the climb"
Half way up or down is to simplify how much (and even if) falling damage applies.
Exception to the general recommendation: if something changes conditions to make it harder. If during the climb, for example, a rock slide starts, or a huge volume of water is unleashed and interferes with climbing - conditions that were not present at the start - a re-check to handle that change is an option.
- If you call for a roll each turn, you will eventually force a failure. It is somewhat penal to the player to do that.
Climbing in general costs half movement
Climbing, Swimming, and Crawling
Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb or a swimming speed and use it to swim. At the DM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. (Basic Rules, p. 67)
Many climbs require no DC/Ability check at all. (A lot of trees are very easy to climb). It is only for the case where the climb is unaided, or has a chance to fail, that assigning a DC and calling for a roll is called for. (Some trees are all trunk until about 30' up: without special tools, they are hard to climb)
References for further reading: DMG p. 237-238; 245