I have some MP3 files with lectures that I need for my studies, and I would like to convert these lectures to text format. How realistic is it to do this over the internet?

Nowadays it is possible, as there are quite a few different services on the Internet that deal with automatic transcription of mp3 to text. Here, for example, is one of them - https://any2text.com/. I usually do this, and these services work pretty well, but you should keep in mind that the accuracy of the finished text they will transcribe for you depends directly on the original audio, how well and legibly it is recorded.