日本語道場 — Nihongo Dojo

～ともなると

~to mo naru to

when it comes to; once it reaches

When to use it

Use ～ともなると when a certain stage, time of year, rank, or scale has been reached and, as a natural consequence, things are qualitatively different. It carries a tone of "as one would expect at that level," so it suits reports, analyses, and commentary rather than casual talk. Common in written business Japanese; ～ともなれば is a near-synonym, and the plainer ～になると lacks the sense of "that's a different league."

Formation

Noun + ともなると — the standard and by far most common pattern; the noun denotes a time, season, age, position, or scale (月末ともなると／部長ともなると／年商百億ともなると)
Verb (dictionary form) + ともなると — possible when the verb names an undertaking or occasion (海外に進出するともなると)
i-adjective / na-adjective — not used; recast as a noun phrase (規模が大きい → 大型案件ともなると)
The consequence clause states a general tendency or inevitability, so it takes non-past forms and avoids commands or invitations
Variant: ともなれば (interchangeable, slightly more literary)

Examples

Once it's closing season, overtime is unavoidable.
Once you're a department head, your decisions carry a different weight.
With a large-scale deal, internal approval takes time.
Once annual sales hit ten billion, audits get stricter too.
When it comes to expanding overseas, that's another matter entirely.