Hello @Blahhitsjazmin!
Those are great questions- everyone has different size candles, so it is natural that they burn at different rates! When you are ready for the spell to end (on the 7th day, since that is what the spell calls for) you can blow out the candles and consider the spell complete. If you still feel a sense of incompleteness, you can say “And now the spell has ended, so mote it be”- although this is optional. As with any spellwork, it is your intent that is the most powerful and potent controller of the spell!
In regards to burying materials, there is a helpful guide here: Disposal of Spell Remains that can help you figure out the best way to go about burying or disposing of spell remains.
Best of luck with the career spell- I hope it all goes well! Blessed be