when can you use the red mana from attacking with radha heir to keld?

[MagicMagic]

when I attack with Radha, Heir to Keld, until what point can I use the two red mana before it is lost?

[Kenji]

Radha's triggered ability gives you the mana "whenever Radha, Heir to Keld attacks." Specifically, this is when you declare Radha as an attacker during the Declare Attackers Step. Mana in your mana pool empties at the end of every phase and step, so you have until the end of the Declare Attackers Step to use the mana.