How it's worked out
It's measured two ways. Looking back, it's the months from start date to the first month at full quota, or to the first of several in a row, since one good month can be luck. Looking forward, it's the ramp a company plans, and it sets what a new rep is expected to sell each month until then.
A planned ramp usually starts from the sales cycle. A rep can't close deals faster than deals close, so a team with a three-month cycle can't expect a full number in month two. New reps often get a reduced quota while they ramp, called a ramped quota, and some plans pay a guaranteed draw alongside it.
Worked example, with made-up figures: full quota is $50,000 a month and the plan ramps over four months at 25%, 50%, 75% and 100%. The new rep owes $12,500 in month one, $25,000 in month two, $37,500 in month three and $50,000 from month four. That's $125,000 across the ramp, against the $200,000 a fully ramped rep would sell in the same four months. Each new hire costs the team $75,000 of sales before they're up to speed.
Activity ramps faster than revenue. A new rep can reach the team's call and meeting numbers in week three, long before the deals from those meetings close. Watching activity during the ramp shows whether it's on course while the revenue is still weeks away.
Where it shows up in your week
A new rep's first months are when they most need their manager's time. Sales coaching preps each 1:1 in a minute, keeps the notes on the rep's page and sends the action items to their task list.