Umbraco - xslt variable into data attribute

"Attribute Value Templates" are your friend here

<p class="time" data-time="{current()/eventTime}">
   Duration: <xsl:value-of select="current()/eventTime" /> hour(s)
</p> 

The curly braces indicate that this is an Attribute Value Template, and so contains an expression to be evaluated.

Note that an alternate way would be to use the xsl:attribute element

<p class="time">
   <xsl:attribute name="data-time">
       <xsl:value-of select="current()/eventTime" />
   </xsl:attribute>
   Duration: <xsl:value-of select="current()/eventTime" /> hour(s)
</p> 

This is not so elegant though. You would only really need to do it this way if wanted a dynamic attribute name.