The first hire is the point where a business stops being one person working and becomes an organisation. The timing question has a reasonably concrete answer.
The signals it is time
Turning down work you could deliver. This is the clearest one – revenue you are declining is the cost of not hiring.
Spending most of your time on work that does not require you specifically. If administration and delivery consume the hours you would use to grow, that is what to hand over first.
Consistent demand over months rather than a busy period. Hiring into a spike and having to reverse it is expensive and damaging.
What it actually costs
More than the salary. Employer contributions, insurance, equipment, software licences, and the time spent recruiting, onboarding and managing.
A reasonable rule is to budget substantially above the salary figure, and to have several months of it available before starting.
The alternatives to consider first
Contractors for specific work, which is flexible and appropriate where the work is genuinely project-based.
Part-time, which is a smaller commitment and frequently sufficient.
And automating or eliminating the work, which is worth checking before hiring someone to do it.
What to hire first
Usually not a version of yourself. The highest-value first hire takes the work you are worst at or least suited to, freeing the hours where you are most valuable.
The mistake to avoid
Hiring because the business feels like it should have staff. The test is whether the role pays for itself within a reasonable period, and if you cannot articulate how, it is early.