I read some good advice on this 30 years ago:
- Make your best estimate.
- Multiply with 3.14.
- Change to the next higher units.
So an estimate of 1 week would in reality be 3.14 months.
On a somewhat more serious note: To make a good estimate, you need to subdivide the project into small enough parts. The parts should be small enough to make it possible to estimate each of them with confidence. If there is a part you don't know enough about, then you have to revert to the formula above.