Can Udyr spend his buff whenever or only on his turn?

Ruling: Udyr's ability to spend his buff is an activated ability without the reaction or action keyword, so it can only be activated at base speed during your turn while there's no chain or showdown.

Nuances:

• This applies to any permanent with "spend buff" templating in the "cost: effect" format for activated abilities
• Spells that say "spend buff" follow different rules and are not restricted to base speed