Several factors affect the exposure time when making a print. One is the brightness of the lamp, some can very slowly start to fade as they get older. Second is the distance of the lamp from the paper, which varies depending on enlargement size. Then there's the speed of the paper, the density of the negative and the aperture you've set the lens to.

To save wasting a sheet of paper you can make a test strip using a smaller piece of the same paper first.

1 Set everything up as you intend to print - height of enlarger head on column, stop the lens down a couple of stops to the optimum aperture, usually around f/5.6. Focus the negative on the baseboard.

2 Then switch off the room light and turn on the safelight.

4 Place the sheet on the baseboard in a suitable place on the print. Try picking a spot where there's a good range of tones to assess when you've made the test strip.

5 Now get a piece of black card that's bigger than the test paper and hold it above the paper so that it prevents any light reaching the paper when the enlarger is switched on.

Tests strips can also be used to check how long you should burn in areas such as sky or a white dress and how long you should dodge to maintain detail in shadow areas.
When you are more experienced try using the f/stop printing method as explained in the feature in May's darkroom section.