  While being decentralized, secure, and reliable, Bitcoin and many other
blockchain-based cryptocurrencies suffer from scalability issues. One of the
promising proposals to address this problem is off-chain payment channels.
Since, not all nodes are connected directly to each other, they can use a
payment network to route their payments. Each node allocates a balance that is
frozen during the channel's lifespan. Spending and receiving transactions will
shift the balance to one side of the channel. A channel becomes unbalanced when
there is not sufficient balance in one direction. In this case, we say the
effective lifespan of the channel has ended.
  In this paper, we develop a mathematical model to predict the expected
effective lifespan of a channel based on the network's topology. We investigate
the impact of channel unbalancing on the payment network and individual
channels. We also discuss the effect of certain characteristics of payment
channels on their lifespan. Our case study on a snapshot of the Lightning
Network shows how the effective lifespan is distributed, and how it is
correlated with other network characteristics. Our results show that central
unbalanced channels have a drastic effect on the network performance.
