Make a rough estimate in your head (in less than 30 seconds) of how much you think it should take you to do a feature, then multiply that by 3 or 4.
Chances are, there will be some tiny details that take a lot more time than you originally thought.
If you think in your head that you should be able to do something in a week, say it will take you a month. If you get push back, resist, and in the worst case say you cannot reasonably do it in less than 3 weeks.
If you do finish it in one week as you expected (highly unlikely), well done: you've under-promised and over-delivered. Management should be happy.