Use of abbreviations and numbers varies in different fields. See a typical example from a biochemistry textbook:

The energy of a green photon…is 57 kilocalories per mole (kcal/mol). An alternative unit of energy is the joule (J), which is equal to 0.239 calorie; 1 kcal/mol is equal to 4.184 kJ/mol.

– Lubert Stryer, Biochemistry

These two sentences demonstrate how useful figures and abbreviations can be; reading the same sentences would be very difficult if the numbers and units of measurement were all written out.

Become familiar with the conventions governing abbreviations and numbers in your field. The following reference books provide guidelines:

MLA Handbook for Writers of Research Papers for literature and the humanities

Publication Manual of the American Psychological Association for the social sciences

Scientific Style and Format: The CSE Manual for Authors, Editors, and Publishers for the natural sciences

The Chicago Manual of Style for the humanities

AIP Style Manual for physics and the applied sciences