Generally speaking, any time you’d want to use a different ad, LP, and/or AG-level targeting option. Also can be good if you want better visibility into the performance of a certain segment.
For an AC company you probably want to have different ad groups for repairs, replacements, and tune-ups as you will want to message differently to those. Also good to separate heating vs AC vs HVAC, and you also might consider separating long-tail vs general keywords since those will perform very differently in terms of CPC and conv rate.
Some skills/tips to make it easy to manage a lot of ad groups:
– Use Google Ads Editor to build in bulk
– Learn to use Text to Columns and Pivot Tables in Excel. More advanced is to learn VLOOOKUP, and create reusable reports that only require you to copy/paste a raw report to update.
– Use a consistent naming convention to facilitate easy filtering & Text to Column usage
– Use labels at the ad group level to combine data across similar ad groups, and create reports based on these labels which can be used to get digestible high-level insights.