A heck of a lot of time.
The sorts of chips that dogs have are passive devices, that are "read" when you put a reader wand near the dog, and you need the reader to be no more than 50cm from the chip.
This is because they are sited for life and have no battery. They just respond if illuminated by a reader.
For something to transmit a location, it needs a battery or other power source. Something big enough to do the job can't really be implanted (and may not even be safe)
If you voluntarily wear a GPS watch or carry a smart phone, then your location can probably be found because they transmit. But even for those with the very longest battery life, and someone who isn't using many functions, you're looking at a couple of weeks tops.
That she left her phone suggests to me
a) she dropped it and hadn't noticed - unlikely as she was on a call
b) she left it on purpose - either because she was doing something she expected to be brief (like scoop poop), or because had decided to vanish (voluntarily walking to new life, or ending hers) did not want to be tracked, but did want dog to be identified
c) foul play