Would probably go out to August here (54 days) ... . Looked at through the lens of what the short straddle is paying as a function of share price, it looks like I should be selling premium in XOP (20.2%), followed by EWZ (17.7%) and/or GDXJ (17.7%).
EWZ offers both better better premium as a function of stock price than IYR at the moment, as well as slightly higher yield (3.66% for the former; 3.50% for the latter). Since I've already laddered out IYR, I may dip at the EWZ well with the 16 delta short put paying .70 in August at the 22 strike, .84 in September at the 21 ... .