#!/bin/bash

sleep 5000

secs=$SECONDS  # utilise bash builtin SECONDS
hrs=$(( secs/3600 )); mins=$(( (secs-hrs*3600)/60 )); secs=$(( secs-hrs*3600-mins*60 ))  # integer maths gives whole numbers

printf 'Time spent: %02d:%02d:%02d\n' $hrs $mins $secs

# output would be
# Time spent: 01:23:20