Question on flight/flying

I've searched the core rules and this forum, but I can't find any hard specifics.  If there is nothing specific in a spell or monster's description, how fast does someone fly? Is it their movement speed? Dash?

Second, how high can one fly? In one of the older AD&D editions the DMG simply said they couldn't fly to high or it would difficult to breathe. 

Thanks for your responses or links to where I may have missed the discussion in my search. 

A creature with a Fly Speed can move this way on it's turn in combat or Travel Pace while traveling outside combat.

There's no altitude limit in the core rules.

Quote from MPA >>

I've searched the core rules and this forum, but I can't find any hard specifics.  If there is nothing specific in a spell or monster's description, how fast does someone fly? Is it their movement speed? Dash?

Second, how high can one fly? In one of the older AD&D editions the DMG simply said they couldn't fly to high or it would difficult to breathe. 

Thanks for your responses or links to where I may have missed the discussion in my search. 

Creatures that can fly will have a Fly Speed listed in their stat block.

Spells like Fly and other effects that grant flight will say explicitly what Fly Speed they give the thing they're targeting. In many cases it's the same as their base Speed, but not always.

The rules don't specify any limit on how high anything can fly.

Quote from MPA >>

I've searched the core rules and this forum, but I can't find any hard specifics.  If there is nothing specific in a spell or monster's description, how fast does someone fly? Is it their movement speed? Dash?

Second, how high can one fly? In one of the older AD&D editions the DMG simply said they couldn't fly to high or it would difficult to breathe. 

Thanks for your responses or links to where I may have missed the discussion in my search. 

Creatures that can fly will fly at a specified fly speed. This can be anything from a set value (not always the same as their walking speed), equal to their walking speed, or based on some formula. Look at the specific source of flight to see what the speed is. Unless specified otherwise, the amount of flying movement can be increased using the Dash action. Any modifications to your Speed that do not specifically modify only one form of movement will modify all forms of movement. Haste will double your walking and flying speed.

There is no hard limit on how high you can fly but check the individual source of flight to see if there is one specified. Many sources will have a set duration and may have consequences if the effect ends and you are not on the ground. For example, Fly lasts for 10 minutes and gives you a fly speed of 60. This allows you to fly 6,000 feet or 12,000 feet if you Dash every turn. However, at the end of that duration, you will fall so you may want to halve that height if you need to survive the return trip.

If duration is not an issue, the only guidelines for atmosphere that I know of are in the Astral Adventurer's Guide (part of Spelljammer: Adventures In Space). In that, objects have a breathable air envelope equal to the longest dimension of its form. The Earth is 7,926 miles in diameter so it would have an air envelope that extends 7,926 miles in every direction from the surface. While in reality the air would thin the further away you are from the gravitational center, the Air Envelopes in Spelljammer are 100% until they're gone. (They do have a measurement of air quality but that's different; the air doesn't thin out.) If you like, you can use this for the maximum height for breathable air.