In APA style, it is appropriate to use numerals (numbers) to express time in a paper. The following examples from the Publication Manual of the American Psychological Association may all be correct:
1 hr 34 min
at 12:30 a.m.
2-year-olds
scored 4 on a 7-point scale
"Exception: Use words for approximations of numbers of days, months, and years (e.g., about three months ago)."
For more information, see APA manual sections 4.27 to 4.33 (approximately p. 108-112)
