As I remember it, most "Frying Tonight" signs also indicated the time they would be serving (usually 6 or 7pm).
The cost of fuel to keep the fat/oil hot meant that many fish and chip shops in small villages or suburbs only opened for an hour. There was also the danger of fire from the big cauldrons they used, so opening hours were restricted. Many also only opened on a Friday (payday) and Saturday.
I remember our local fish and chip shop in the early 60s was a small cottage with a "stable" type door. There was no sign to indicate it was a fish-and-chip shop, just a blackboard put out on Friday and Saturday afternoons with the words "frying tonight" and the time.
At the appointed time there would be a big queue waiting, perhaps up to 50 people. But because the only choice besides fish and chips was a pickled egg (no pies, saveloys, kebabs, gravy or sauces) people were quickly served and the queue would have gone within 20 minutes. There were always a few teenagers hanging back at the end of the queue to get newspaper cones of the leftover batter "bits".
I remember peering over the stable door to see two big gas rings on the floor, connected to the overhead gas-mantle by rubber tubing, each under a cauldron of bubbling fat, one for chips, the other for fish. The lids were kept handy to slap on the cauldrons in the event of fire, and the blackened walls and ceilings showed that was not an unusual occurrence.
The old chippie closed not long after a brand spanking new one opened in the local parade of shops. That had the new-fangled stainless-steel range that could keep the fat/oil bubbling away for hours economically and also had the warming tray to keep pies and saveloys hot.
The last times I saw a "frying tonight" sign was in a small village in the Derbyshire Dales in the 1980s, also in the 1980s the Fish and Chip Shop in St Mary's in the Scilly Isles also only opened for an hour or so and had a "Frying Tonight" sign with the time. Strangely the last time I saw a "Frying Tonight" sign was in 2004 in Reyðarfjörður on the East coast of Iceland. They had picked up a liking for Fish & Chips from the British Servicemen serving their during the War.