Starting Formula milk

Ftm here. I'm starting fm for my 4m 20D infant as I'm returning to work soon. Baby has the tendency of not finishing her milk at 1 go, eg. Will finish up frozen ebm within 2 hour frame thus I wanted to check: 1) how long can the fm be kept for if baby unable to finish the milk at one go? SG fm is so expensive😔 2) are we able to keep the fm more than 3 weeks after opening? Eg. 4 weeks 3) will you use a thermometer to measure the boiled water's temperature before preparing milk? Thank you very much! #pleasehelp #advicepls #firstmom #firstbaby