I believe there is a master thread for upgrade chances using a SWU, but the general guideline is to factor in the day of the week of travel, time upgrade was requested and added to WL, status of those traveling and the number of unsold seats in business class shortly before the date of flight. Your parents will have no status, so probably be lower on the upgrade list than EXP, PLT and GLD on the wait list and some passengers may be on the WL using miles+copay.