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In general, longer developing time will both increase the overall contrast and exaggerate the appearance of grain in the negatives. The longer the film is in the developer, the greater these two effects will become.
Less dilute (stronger) developer solutions will be similar in effect to longer development times. Meaning, both the contrast and grain will increase. More dilute (weaker) dilutions will have the opposite effect. Shorter developing times are usually used with stronger dilutions, longer times with weaker dilutions.
In general, yes. Especially when unadjusted longer times are used. Over time you will discover your own tipping point, beyond which the increased grain is not to your liking.
Not necessarily. Most agitation cycles are usually intermittent. Something on the order of 10 seconds agitation for every 30-60 seconds of elapsed time. The greater the percentage of time spent agitating, the less overall development time is required. They are a trade-off.
Be aware that temperature can have a large effect on the outcome as well. Try to keep your developer temperature reasonably consistent (within a degree or two for each developing run). The standard is 68F/20C, but others can be used with adjustments to developing time.
Also try to keep all of your solutions (developer, stop bath, fixer, clearing bath if you use one) within a degree or two of each other. This includes the wash water at the end. Too much of a difference between solutions can physically damage the wet film.
Be gently wary of the advice you might get here. Many have been doing this for so longdecades in some casesthat their answers, while well meaning and offered in good faith, will unwittingly expect you to have more unspoken background knowledge that you may have at this point.
The advice they give will be for the most part correct, but it still may not make sense. So don't be afraid to ask for clarifications. Every single person here was in your same shoes at some point in their past.
Also, consider experience to be your best guide and teacher. Once you have enough informationor think you doto try something out, just do it. Keep track of what you did. Write it down if necessary. Then examine your results and adjust as needed. Keep in mind that failure is your best teacher. Embrace it whenever you can. It's presence means you're making good progress.
If (when) you get stuck, just wander back here and ask for more advice. You'll get tons...
Ken