Protection from lightning is only ever going to be partial. You can always have a stronger strike or a nearer one that will kill your device.
Practically, presume that a direct strike *will* destroy your device. Then figure out how much protection is economically viable given the probability of the event occurring, and the potential cost of the damage.
If the device is connected 24x7 and you're in a lightning prone area, and something expensive is connected, and it's in a location that is more likely to be close to a strike, spend more.
Typically you would use a TVS device rated higher than your normal mains voltage and with an energy rating as high as possible given your budget. TVS devices generally have a current limit, but the energy limit (say 100J) tells you how long they can last at that current. e.g. a 1000A 100V 100J device could withstand that 1000A with 100V across it for 1/1000 of a second. that's 100J/(1000A * 100V)
Another device to consider is a gas arrestor or a spark gap.
In many cases these devices will suffer when called on, so depending on the energy they are asked to absorb they may be unaffected, slightly degraded, effectively destroyed, or insufficient leading to damage to the protected device. Note that eventually you get to the final state as cumulative damage leads to failure at a lower energy level.
Another factor is that energy from a lightning strike can enter from multiple sources. It might be seen as a huge spike in mains potential, but it can also result in a potential difference between neutral and earth, or it can come in via your outputs (say from an antenna), or it can just be inductively coupled to some of your wiring.