Many things will depend on the destination country. But in general, once the documents are submitted, it may take about 2 weeks to get the decision. This period can be extended to 1 or 2 months but personally I have never experienced this (but I am not an Indian citizen so YMMV). Again, depending on the destination country, the toughest part may be getting an appointment to submit the documents. The closest available dates may be several months ahead.
For a touristic trip, they could ask for a proof of residency, employment details, itinerary/hotels/air ticket reservations (usually no need to buy, only reserve), financial means, health insurance, and photos.
Agents are usually not required, you can submit the documents by yourself. Sometimes, directly to the embassy/consulate, sometimes they ask you to submit the docs through an intermediary like VFS. Again, depends on the destination country.