Depending on the finetuning tool you're using, you can just start the training run, and then it shows you how long it'll take. Like give it 5 mins to stabilise, then see the estimated duration.
Axolotl is a good finetuning tool if you need one.
I really wish people developing local applications would allow users to specify an API endpoint. Most applications use an OpenAI compatible API, and if they don't the browser's implementation of local model inference can be used.