Recording time calculator
A 128 GB card holds about 2h 51m at 100 Mbps, but only if it can sustain the write speed. Camera bitrates are quoted in megabits and card speeds in megabytes, so a 100 Mbps recording needs 12.5 MB/s sustained — well within a V30 card, and a problem for an older class 10 that only guarantees 10. The V-rating exists precisely because burst speed and sustained speed are different numbers.
Recording time is card capacity in bits divided by bitrate. A 128 GB card at 100 Mbps holds about 2 hours 51 minutes — roughly 45 GB per hour of footage.
How to work out recording time
Two things shorten the real figure below the calculation. Cards are sold in decimal gigabytes and formatted capacity is a few per cent lower, so a "128 GB" card offers around 119 usable. And write performance degrades as a card fills and as it ages, which on a marginal card can cause a recording to stop mid-take. Professional practice is to swap cards at around 80% rather than filling them, which costs a little capacity and removes a whole class of failure.
Questions
About 2 hours 51 minutes at 100 Mbps, or 1 hour 25 at 200 Mbps.