How reliable is Firestore as an offline persistence mechanism?

On Android (as of this writing) Firestore uses SQLite as a persistence mechanism. So for intermittent periods of offline activity you should have no problems with performance or durability.

However if you are going to be offline for days or weeks (as you said) there are some things you should be aware of:

Performance

Because Cloud Firestore is meant to be used mostly online, pending writes that have not yet been synced to the server are held in a queue. If you do many pending writes without going online to resolve them, that queue will grow and it will slow down your overall read/write performance. Most of Cloud Firestore's performance guarantees come from indexing and replication on the backend, but most of those optimizations don't exist when you're operating offline-only.

Conflicts

Firestore's basic conflict resolution model is "last write wins". So if you have many offline clients writing to the same document, only the last one to come online will actually "win" and persist their change.

Features

Most of Firestore's features work offline with one major exception: transactions. Transactions can only execute when you are online. So if your app uses transactions it will not work properly offline without some special handling.

There is no indication in offical documentation on how durable the offline persistence is because it cannot be predicted. This question cannot have an exact answer, like 4 weeks or something like this because it depends on how many write operations take place while you are offline.

I recommend you not to use Cloud Firestore as an offline-only database. It's really designed as an online realtime database that can work for short to intermediate periods of being disconnected.

While offline, it will keep queue of all your write operations. As this queue grows, local operations and app startup will slow down. But you need to know that these operation will persist even if you restart the device. You not gonna lose any data.