If you apply for Diploma in Aeronautical Engineering via AME CET the full admission process usually spans a few weeks to a couple of months.
First you fill the application form (online or offline) when registration window opens.
Then you take the AME CET exam, which is typically conducted soon after registration ends.
After results are declared you go through counselling and seat allotment based on your rank.
The exact time depends on when the registration starts exam date result declaration, and counselling schedule of that year. Because different institutions may follow slightly different dates the process may take less or more time.
If you apply properly and on time you can complete the process fairly quickly. You will know whether you got admission within a few weeks. This gives you enough time to plan your studies or backup options.
Sometimes result announcements or counselling schedule might get delayed. Or maybe application window begins late. But even then you can stay updated on official site of AME CET and ensure documents are ready so you do not lose seat.