The question hides three different clocks. How long the return takes you to complete depends almost entirely on how the year's records were kept. How long HMRC takes to process it is usually short and largely out of your hands. How long a refund takes is a third clock again, and it only starts once the first two have finished. This page gives realistic timings for each and, more usefully, names the things that actually make a return slow.
Filling it in: hours if the records exist, days if they do not
For a straightforward sole trader whose income and expenses are already totalled, the form itself is an hour or two of typing. What consumes the time is the reconstruction: finding twelve months of receipts, deciding which payments were business costs, chasing a CIS statement that never arrived, or working out which side of 5 April a payment fell. Somebody who has kept a running total finishes in an evening; somebody starting from a carrier bag of paper should plan on days, and should not start in the last week of January.
What makes a return genuinely slow
Waiting on a UTR that has not arrived, waiting on a P60 or a contractor's statement, an income source that needs a supplementary page you have not filled in before, and any figure that requires a decision rather than an addition. Each of those is a wait on somebody else or on a judgement, and none of them speeds up because the deadline is close. They are also all foreseeable in October, which is the argument for starting then.
HMRC's side, and the refund
An online return is normally processed quickly, and the calculation appears against your record. A repayment then follows, usually within a few weeks, sometimes faster, and it can be delayed if HMRC runs a security check or if the bank details on the record are stale. A repayment is also set against anything else you owe, including a payment on account that is about to fall due, so the figure that arrives can be less than the overpayment. Paper returns are slower at every stage.
The deadline is fixed whatever the timings
None of this moves the date. An online return for a tax year is due by 31 January following it, and a return filed late attracts a £100 fixed penalty even when no tax is owed and even when the delay was caused by waiting for somebody else's paperwork. Filing on time with a figure you will later amend is almost always better than filing late with a perfect one, and the amendment window gives you 12 months after the deadline to correct it.
Questions people ask about how long does a tax return take
How long does HMRC take to process an online return?
Usually quickly - the calculation appears against your record soon after filing. Paper returns take considerably longer at every stage.
How long until a refund reaches my bank?
Typically a few weeks after the return is processed, assuming your bank details are current and nothing is set against the repayment.
Can I file now and fix a figure later?
Yes. You can amend a filed return for 12 months after the 31 January deadline for that year, and filing on time then amending is far cheaper than filing late.
Is it quicker if an accountant does it?
Not necessarily quicker to process, but it removes the decisions from your evening. The processing time at HMRC's end is the same either way.