This depends on the altcoin-specific mining implementation. However, assuming a mining model like Bitcoin, which runs on proof-of-work, you'd get the majority of coins mined in any given day if you have a higher hashrate than the rest of the network combined.
So, if there are 100 coins available to be mined per day, and before you start mining the network hashrate is 100 hashes per day, you'd mine 33 coins per day if you suddenly started mining at 50 hashes per day, assuming difficulty is adjusted on a per-block-basis.
Or, if the network hash rate is 40 MH/sec and you suddenly start mining with 220 MH/sec, and the number of coins issued stays stable, you'd get 84.6% of the coins mined, on average. This assumes the proof-of-work required to generate a block of coins is recalculated continuously, which is not generally the case, but it makes the calculation easier, and at some point the network will adjust the difficulty to reflect the total hashrate. If this was Bitcoin, where the difficulty is adjusted every 2016 blocks, you'd have a major advantage for up-to 2016 blocks, or whenever the next difficulty adjustment occurred.