I'm using the get_transfer RPC method.
I'd like to know what the maximum amount of transactions this API can return? Can I filter it using timestamp or pagination instead of block height?
As per the documentation you linked to, you can filter by
account_index/subaddr_indices, not timestamp. As Monero blocks are mined roughly every 2 minutes, you can use heights as an approximation of time.
There is no restriction imposed as to the amount of transactions returned other than by the filter options.