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Hi @Ghatikara, rescheduling your appointment costs $250 each time. You have sufficient time for May 2021 L3 as it is still early. Typical candidates use about 5-8 months for preparation, targeting 300-350 hours of study time. Depending on your personal circumstances and time commitments, e.g. if you can commit to 14 hours of study a week, 6 months is sufficient, if less, the study time is longer obviously.
Since it is still early, you could definite start studying now and be committed to prepare for May 2021, avoiding the issue altogether. We all never feel quite prepared at the end, but definitely ready to have a go at the exams (as a result of being fed up of studying)!