Relevant text:
Awaken. Duration: Instantaneous. …After spending the casting time tracing magical pathways within a precious gemstone, you touch a huge or smaller beast or plant.
It doesn't say anything about a span of time after the agate is carved, or needing to be near the agate at all for the spell to take effect. Taking this to the extreme, could you spend 30 years carving agates, then walk through a forest barefoot with your stored agates across the world. and create an instant horde?
Best Answer
You only have an instant to touch the plant or creature
Awaken has a casting time of 8 hours, and consumes an agate worth 1,000 gp as the material component. As per the spellcasting rules (Casting a spell, p. 202 PH:
So anything in the description is the spells effect, and the effect only happens once the spell has successfully been cast. This can cause issues, when the description covers activities that sound as if they should be part of the spellcasting, not the effect, like in this case (burning hands is another good example). Here the text in question is:
This does not say immediately after spending the casting time, so you could think that technically any time after spending the casting time qualifies, even if it is 30 years afterwards.
However the spell has a duration of Instantaneous:
The entire effect is instantaneous, and it happens as soon as you finish casting the spell. Because of that, you only have an instant to touch the plant or object, not 30 years.