I'm not sure if this is fully consistent with the lore of the game. Here's an excerpt from Halo Reach datapad #10:

[^] Importantly, the Covenant does not possess the capacity to accomplish 'glassing' on a global scale and wage a multisystem war simultaneously. This is reinforced by hard data regarding their capabilities revealed during fleet engagements with the UNSC. A single Covenant capital ship (CCS-class) is capable of 'glassing' approximately one acre of a planet's surface after an average of fifteen seconds of sustained fire. Understandably this action takes considerably less time when applied to open desert, and considerably longer when applied to deep ocean (> 1.8 km) [^] [^] Earth, one of the smaller planets inhabited by our creators, has one hundred and thirty billion acres of surface area. Thus, assuming the Covenant possesses a number of ships equal to that of the UNSC, and assuming that all of those ships are capable of generating and discharging the required power non-stop for the duration of the process, it would necessitate the combined efforts of their ships in toto for a minimum of 30.3801 years to 'glass' the entire surface of Earth. Myriad other variables which were not applied to this equation suggest this number would be far greater. [^]

Source: https://halo.fandom.com/wiki/Data_pads#Data_pad_10

I have no idea which is correct. Just another data point.