A good pitch deck is important for the serious entrepreneur. You and your team should focus enough time on your deck to ensure you have covered all essential areas:
- Problem + Market/Opportunity;
- Your Solution/Benefits/Product;
- Team + Advisory Board;
- Financials (Ask, Cash Burn, Runway, Funding Usage);

There is no telling how long you should invest in creating the pitch deck. When these essential areas are covered, you can consider your pitch deck complete. You should regularly update it based on product/financials changes and investor feedback.

And yes, you should have your pitch reviewed by an investor if there is one available. Be aware, though, that only investors that might realistically invest will give valuable feedback. If you ask feedback to an investor that is not in your area or is not interested in your project, he will give you "generic" feedback that might not be useful. What you want is specific tailored feedback from investors that truly match your company/product.