What algorithm is appropriate for a thermostat controlling the heating of a room with floor heating?
Maybe approach it from a "BTU's needed over time" standpoint. I don't know the exact maths off the top of my head, but consider: 


What is the temperature differential? 
What is the heating capacity (BTU's) of the radiant flooring system? 
How long does it take to start rendering 68% of that heat to the room? 
What is the heating load of the room in BTU's? 
How efficient is the heat generated?


Then do some math on these numbers. Something like...

$t_{hours}=\dfrac{(T_{end}-T_{start})\cdot H_{demand}}{H_{supply}}\cdot \mathrm{eff}^{-1} + t_{delay}$

So for an end temperature of 22°C and a start of 18°C, heat of 50kBTU, demand of 20kBTU, 90% efficiency, and 15 minutee delay:

$\begin{align*}t_{hours} &amp;= \dfrac{(22°C-18°C)\cdot 20k_{demand}}{50k_{heat}}\cdot 0.90_{\mathrm{eff}}^{-1} + 0.25h_{delay}\\
&amp;= \dfrac{80\mathrm{k}}{50\mathrm{k}}\cdot 1.\bar{1}+ 0.25\mathrm{h}\\
&amp;= 2.02\bar{7}\mathrm{h}
\end{align*}
$

Some analysis and experimentation will be needed to arrive at realistic values but hopefully this is a start.