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.