100 gallons is a nice milestone. Personally I'd commemorate the number of batches, so you should be coming up on 25...another reason to celebrate!
How I see my own brewing career progressing:
25 batches--solid understanding of brewday basics (mashing, boiling, chilling, pitching), knowledge of several styles with incisive grasp of a few styles, basic knowledge of several types of malt and hops, basic understanding of recipe development, use and understanding of a few yeast strains, basic understanding of fermentation (making starters, understanding pitching rates, fermentation temperature control), ability to hit ballpark OG and FG
50 batches--advanced knowledge of brewday basics (multistep mashing, decocting, precise temperature control on mash), incisive knowledge of a handful of styles, independent/stylized approach to brewing, knowledge and understanding of all base malts and some specialty malts and several hops, ability to independently develop recipes in a few styles, general knowledge of yeast strains including selection of house strain(s), solid knowledge of fermentation (precise pitching rates, pitching rates by style, appropriate selection of strain, washing yeast, repitching slurry), ability to hit gravities within a few points on most occasions
100 batches--generally flawless brewdays with the very occasional small hiccup, incisive understanding of a large number of styles, knowledge/understanding of all malts and hops, completely independent development of recipes in a large number of styles, experience with most or all yeast strains, advanced knowledge of fermentation, ability to hit gravities on almost every batch
250 batches--always flawless brewdays, independent development of recipes in any style, ability to hit gravities precisely on every batch
The above schedule presumes a somewhat scholarly approach to brewing (i.e. trying to learn new things, broadening your horizons beyond a few styles, reading books on the subject) rather than grinding out batch after batch of the same brew for months on end. Brewers might have some of the abilities earlier or later depending on their specific interests--for example some people are really into hops, others into yeast, others into mashing so they will be ahead of the curve in those areas at an earlier stage in their brewing career.
I will be there soon enough. I plan go brew my limit this year. At least get damn close...
