Transaction still listed as 'pending' after 12 hours
The higher the number of unconfirmed transactions (in the mempool) the longer will take to confirm your transaction unless you give a higher fee.

I recommend watching current mempool where you can see the number of unconfirmed transactions and it gives you an idea how "clogged up" is the Bitcoin network.

Unconfirmed transaction will either confirm or expire (not sure but I think it takes 3 days to expire).