Found this from Shivohum2015,

Reputation in steemit is purely the amount and not the number of votes....no matter whether you get 10 votes or 100 votes or 500 votes it is the weight of the vote, the effective vote in figures that is showing as the earning of the post(unpaid) will count to your reputation.

From 25 to 30 reputation -- it is very easy and you need vote of roughly 2 sbd/usd worth upvote.

From 30 to 40 reputation -- It is also easy and you need around 6 or 7 sbd/usd worth upvote.

From 40 to 50 reputation -- It is not that easy and it is difficult as compared to previous level and at this stage you roughly need 35/40 sbd/usd worth of upvote.

From 50 to 60 reputation -- Very difficult and it will take some time as you need around 400/500 sbd/usd worth of upvote.

From 60+ reputation onward -- It is very very difficult and it will take even more time than previous and you will need more and more upvotes to keep your reputation increasing.