How long should I wait for an amended return (which was mailed on 3/26/16) to show received before sending the amended return again?

I believe a mailed return is 6 weeks.

An amended return can take up to 16 weeks (4 months) to process.     You should always mail certified with return receipt as proof of mailing and received by IRS.  Do not mail a 2nd amended return, having 2 amended returns at the same time can delay things even more.

You can check the status of your mailed amended return at this link but allow 3 weeks after mailing before checking.
    http://www.irs.gov/Filing/Individuals/Amended-Returns-%28Form-1040-X%29/Wheres-My-Amended-Return-1