The laddering happens when your income drops, which is usually after retirement. You can only put $5500 a year per person into an IRA (barring higher limits for older age people). So, anytime you're in the 25% bracket or higher, you contribute to the tIRA to bring down your AGI. After retirement, when you have less income, you can convert from your tIRA to Roth IRA. This counts as income against your taxes, so it costs you money to do this, but then you can grow it tax-free forever in the Roth. You can't contribute more than $5500 a year in total.
Regarding your withdrawal question, for a Roth IRA, you have two time limits to worry about. First, earnings and interest has to age for five years, and that clock restarts anytime you make a conversion. This is where the "laddering" concept comes from. Then, if you want to access the money before you're 59.5, then you have to use the SEPP/72(t) tactic. There are some great posts on this here in the Investing forum. This lets you retire early and spread out the payments until you're 59.5, at which point you can withdraw whatever you want, tax free.
