I would set the duration to 'instantanious' automagically because this is inherently a kind of ascension spell. It uses the transform seed as its base (obviously), but the change really isn't magical after it has been completed.
Also, from a flavor aspect, item creation cheese aside, thousandth-level characters are insanely ridiculous.
As an example, I have a 120th-level archmagess in the world I designed for WotC (crossing my fingers there

- but she's nearly 4,000 years old, cannot remain in the world much longer, is already a demigoddess, and beyond stating her level, she is not actually a statted character.
So, I would try to avoid forcing DCs into the thousands. Indeed, I'm halfway tempted to change the XP penalty from 1 DC per 100 to 1 DC per 10. Still not sure if it's balanced, but it seems to make so much more sense.
Either that or drop the base of 10 for seeds to a base of zero, with a 1-round casting time (increase rounds to minute, up to 10 minutes, than hour/hours, then days up to a year and a day, for time-based mitigating factors).
Maybe both
