If you are acting under an in-country right of appeal, you will typically need to lodge your appeal with the First-tier Tribunal within 14 days of the Home Office’s decision on your original immigration or visa application.
However, if you are instead taking advantage of an out-of-country right of appeal, your appeal will usually have to be lodged with the First-tier Tribunal within 28 days of the Home Office decision. If you are required to leave the UK before you can lodge your appeal, the deadline for submitting your appeal claim will be 28 days from the date you leave the country.
While it is technically possible for appeals to be sent outside of these specific time limits, the First-tier Tribunal is unlikely to be willing to hear an appeal beyond the pre-specified deadline in all but exceptional circumstances.