As the <<timed>> macro's documentation clearly states it only accepts CSS time values, which must include a bundled unit (e.g. ms and s). Thus, you may not simply use an integer, no.
What you can do is to add the appropriate unit to either the value stored within the variable or at the macro invocation via a backquote expression.
Example using a temporary variable to hold the CSS time value:
<<set _delay to (_min * 60) + 's'>>
<<timed _delay>> … <</timed>>
Example using a backquote expression to create the CSS time value at the macro invocation:
<<timed `(_min * 60) + 's'`>> … <</timed>>
PS: Unless you're using the time values in other passages, you should probably be using temporary variables rather than story variables, as the former do not become part of the history.