Thanks for reaching out with this beekeeping question. Formic Pro does sometimes lead to queen loss (see more information below). At this point, it's hard to know if your colony is queenless or if it has a new queen who hasn't started laying yet. Finding a bunch of empty "polished" cells in the brood nest area can be a good sign that your colony is queenright and that the workers are preparing cells for her to lay. 

If you have another queenright colony, you can give the colony without brood a frame with eggs and young larvae and then check that frame in a few days. If the colony builds queen cells on the frame, then it is likely queenless, and you can decide to introduce a new queen or let them raise a new queen. If the colony doesn't build queen cells on the frame with young brood, then it likely has a new queen who hasn't started laying yet. Additional information on troubleshooting queenless colonies is in the Help! I need a queen article.

NOD Global, the manufacturer of Formic Pro, states the following in italics (under product FAQ):

WILL FORMIC PRO KILL MY QUEEN?

During dearth periods, when ambient temperatures are above 29.5°C (85°F), there is an elevated risk of queen loss, supercedure, or delay in egg laying. Treatment should be postponed until temperatures drop or nectar flow resumes. Formic acid will initially disturb colony activities and may, within one day of application, result in queen rejection or slight increase in adult bee mortality. Some brood mortality may occur in the initial stage of treatment.

Natural honey bee emergence and mortality rate is approximately 1,500 bees per day. A one-day equivalent of natural mortality (i.e. 1,200 to 1,500 bees or up to 2 cups) may be observed at the hive entrance during the treatment. Treatment may trigger supercedure of older or poorly mated queens, regardless of age.

Do not destroy queen cells that may be observed prior to, or post treatment. Supercedure, even if thought to be set in motion by treatment, is a natural process, and should be allowed to proceed for the health of the colony. Verify colonies are queen-right one month after treatment. Mother and daughter queens present post treatment is not uncommon.

Happy beekeeping!

Good question! I would check the potentially queenless colonies within about 3-7 days of giving them brood frames from the other colonies. This timeframe should be enough to see if the colonies develop queen cells on the brood frames that they were given but before a new queen could develop and emerge.

Happy beekeeping!