Suppose you're working in software development in the UK, recently graduated and now with a little experience but still quite early in your career.

If you're outside London and living in a shared place, maybe your salary is £30k, and your monthly living costs for rent, bills, and essentials like food and travel are £1,500. Your net monthly income after taxes and student loan repayments is probably about £1,850.

So, you have a surplus of £350 per month. If you set that aside throughout your next year of employment, sticking it in an ISA (a tax-exempt savings account), then by the end of that year you have nearly three months of safety margin.

Suppose, ignoring inflation to make the maths easy, that your salary increases at about 10% that year, without any negotiation or changing jobs. Now your take-home pay is probably about £2,000 after deductions, giving you a £500 surplus each month. If you saved that away the following year, you'd have more than 6 months of safety margin set aside within 2 years of starting to save.

Of course these are just example figures, hopefully not unrealistic for someone early in a software development career in the UK today, and they ignore any one-off major expenses you might have like getting a car serviced and MOT'd once a year or taking a foreign holiday. But even if you do those things, you can surely still save something, and when it comes to having a bit in reserve in case your career hits a bump, anything is better than nothing.

Edit: After checking a few prices, it turns out that some costs of living have gone up much less than I expected since I was that young. :-) So actually £1,500 is probably a lot more than you really need to spend on essentials under the conditions I mentioned, even if you're living in a tech-focussed area, and saving a few months' worth of essentials expenses should be much quicker than I worked out above.