A "rough guess" means making an estimate or prediction based on limited information or without precise calculations. It is an approximate answer or a quick estimation. Full definition
Despite all the fancy retirement calculators available to modern investors, these tools provide at best rough guesses of what may or may not happen in the future. (moneysense.ca)
If the plug finally fell out on a longer trip (rough guess based on my oil changes: maybe as little as 20 - 30 minutes), almost all of the oil could drain out on the road before parking it at home or work, where you would have the possibility of noticing it. (mechanics.stackexchange.com)