Once Ashling leaves the battlefield, you have until the end of the current step or phase to spend whatever red mana you have before you lose that mana as normal. There is no penalty associated with this other than the loss of the mana.

If an effect creates multiple copies of an instant or sorcery spell, Ashling's last ability triggers once for each copy created by the effect.

If a red mana you add has certain restrictions or riders associated with it (for example, if it was produced by the last ability of Arena of Glory), they'll apply to that mana no matter when you spend it.

You can keep unspent red mana indefinitely while Ashling is on the battlefield. That means if you add a red mana during one step or phase, you can spend it during a later step or phase, or even a later turn. You will still lose other types of unspent mana as each step and phase ends.

Some effects instruct you to copy an instant or sorcery card in a zone other than the stack. These copies will not cause Ashling's last ability to trigger. However, most effects that do this also allow you to cast the copy, and casting the copy will cause Ashling's last ability to trigger.