Hey Ramin, how are you? I hope you are doing just fine!
I shall provide you with information on the usage of ''may'' and ''could'' in sentences. Firstly, you should know that ''may'' and ''could'' care not used to express a general truth. When you use either of them, you are only expressing a weak/remote possibility. Here are some examples:
1) ''You should not go there by yourself - it could be fairly dangerous!''
2) ''It may be difficult in the beginning, but I am quite certain you will make it through!''
3) ''If you are to help him with his studies, be aware that you may be wasting your time''.
4) ''Passing that exam could be hard for everyone, even for someone like him''.
Secondly, let's review your text, shall we?
''You should not spend your time ''working'' on any application which is not worth it. It may/could waste your time. I think you should be patient and wait for a project which may/could be more expensive''.
As you can see, you may use either of these modal verbs as you like. In regard to your sentence, the only mistake is that of the usage of the verb ''work'', which should be put in the -ing form, and not in the infinitive form (''to work''). That is because the correct construction regarding the usage of the verb ''spend'' in a sentence is: ''to spend (one's time) + verb (in the -ing form) + something''. Here are a few examples that may help you:
* If someone does something from the beginning to the end of a period of time, you say that they 'spend the period of time doing it'.
E.g.: ''We spent the evening talking about art''.
E.g.: ''I was planning to spend all day writing''.
E.g.: ''We shall spend the time that remains (of us) praying for forgiveness''.
Hope you have understood it! :)
Should you have any further doubts, do not hesitate to contact me. I shall be pleased to help!